**An Extensive Collection of Evaluation Indicators to Assess Occupants' Health and Comfort in Indoor Environment**

#### **Fabio Fantozzi and Michele Rocca \***

Department of Energy, Systems, Territory and Constructions Engineering, University of Pisa, 56122 Pisa, Italy; f.fantozzi@ing.unipi.it

**\*** Correspondence: michele.rocca.au@gmail.com; Tel.: +39-050-221-7104

Received: 26 November 2019; Accepted: 4 January 2020; Published: 12 January 2020

**Abstract:** Today, the effects of the indoor environment on occupants' health and comfort represent a very important topic and requires a holistic approach in which the four main environmental factors (thermal comfort, air quality, acoustics, and lighting) should be simultaneously assessed. The present paper shows the results of a literature survey that aimed to collect the indicators for the evaluation of occupants' health and comfort in indoor environmental quality evaluations. A broad number of papers that propose the indicators of a specific environmental factor is available in the scientific literature, but a review that collects the indicators of all four factors is lacking. In this review paper, the difference between indicators for the evaluation of risk for human health and for comfort evaluation is clarified. For each environmental factor, the risk for human health indicators are proposed with the relative threshold values, and the human comfort indicators are grouped into categories according to the number of parameters included, or the specific field of application for which they are proposed. Furthermore, the differences between human health and comfort indicators are highlighted.

**Keywords:** health and comfort; evaluation indicators; work environments; indoor environmental quality; indoor comfort; human health

#### **1. Introduction**

Nowadays, the awareness of the relevance of obtaining buildings with high performance to reduce both energy consumption and the impact on the environment (CO2 emissions into the atmosphere), have been collectively reached. The challenge is now to guarantee a high level of housing quality while maintaining low energy consumption. People spend almost 90% of their life in indoor environments (houses, schools, work environments, etc.) and the effects of indoor conditions on human health cannot be ignored. The interest in buildings that guarantee high levels of occupant health is increasing both in research and in professional practice. In different areas (i.e., health, economy, etc.), comfort, well-being, and quality of life are becoming increasingly important [1,2]. The built environment can be interpreted as a physical and social environment that must guarantee the environmental conditions to promote wellbeing, health, productivity, and interactions between people. To this aim, at the European level, new building certification assets have already been developed; they include not only the assessment of energy performance, but also the levels of comfort and well-being that can be achieved within the indoor environment [3–6]. Health is defined by the World Health Organization (WHO) as "a state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity" [7]. Corresponding to this definition, there is a change of the public health focus from life expectancy to health expectancy, which no longer includes mortality in favor of aspects of quality of life [8,9].

Indoor environmental quality (IEQ) assessments, with particular reference to work environments, should combine the evaluation of all the possible factors that can have negative effects on health with the evaluation of the perceived levels of comfort [10]. New holistic approaches in which building energy performance is combined with the indoor environmental conditions, are increasingly used [11–14].

The health risks evaluation is generally carried out using "pass/fail criteria" purely quantitative that generally involve comparisons between the measured or calculated parameters and the related limits (threshold values).

The comfort evaluation includes quantitative, qualitative, and subjective investigations. In this field, the human perception is particularly relevant and cannot be neglected. Comfort assessment is generally performed through the evaluation of the IEQ, that is based on four environmental factors that simultaneously affect the human perception, namely: thermal environment, indoor air quality, acoustical environment, and visual environment [1,15–18].

Concerning the effects of the indoor environment on human health, in addition to the building-related illnesses caused by specific exposures in indoor environments (e.g., rhinitis, asthma, and hypersensitivity pneumonitis, etc.), sick building syndrome (SBS) consists of a group of mucosal, skin, and general symptoms that are temporally related to working in particular buildings [19,20]. The SBS is characterized by "non-specific" effects and its signs can be highly variable, affecting diverse parts of the human body, and correlating them to SBS could be challenging in the first place [21]. The SBS was introduced the first time by the WHO in 1983 [22], and it is well demonstrated that sick buildings can also induce stress, anxiety, and aggression [23,24]. These negative effects can further result in an increase in the possibility of hazardous events in workplaces [21].

Another important point is the selection of evaluation indicators. There are numerous indicators in the scientific literature, but the distinction between health and comfort indicators is not always clear. Concerning the thermal environment among the huge amount of indicators proposed in the literature, only five are normalized: Wet Bulb Globe Temperature (WBGT), required clothing insulation (IREQ), Predicted Heat Strain (PHS), and Predicted Mean Vote (PMV) and Predicted Percentage of Dissatisfied (PPD) for the thermal comfort [25,26]. On the contrary, concerning the visual environment, specific indicators for the assessment of the risk arising from a non-adequate lighting exposure do not exist, and the same indicators are used for both visual comfort and improper light exposure assessments. Moreover, only some indicators (e.g., PMV, PPD, etc.) can be used alone because they combine different physical parameters and provide overall information on the human perception of a specific environmental factor. The other indicators (e.g., illuminance, reverberation time, etc.) provide detailed information and it is necessary to combine the results of more indicators to obtain an overall evaluation on environmental factor.

In the literature, articles are available that collect indicators of individual environmental factors (i.e., [27–31]); however, there is no review to collect indicators of multiple environmental factors. For this reason, and given the growing interest in overall assessments, the authors decided to collect the indicators of all four main IEQ environmental factors: thermal environment, indoor air quality, acoustics and lighting. The objective of this paper is to create a comprehensive overview of indicators for assessing human health and comfort in the indoor environment. This collection could also be a useful starting point for those who must approach global assessments of the indoor environment. The present paper is composed of a first section on health and comfort evaluations where some general indications are provided, four further sections, one for each main environmental factor, and the conclusive remarks. The four sections on the environmental factors are developed following the same structure. The first part is related to the risk for human health evaluation and it is composed of an "Overview", a framework of "Guideline and legislative outline", and a collection of the "Indicators". The second part is related to the comfort aspects and it is composed of an "Overview", a collection of "Indicators", and some information on the "Current research trends". For both risk and comfort, the most commonly used indicators are briefly described, and the main aspects discussed. With regard to the risk assessment for human health, together with the most important risk indicators, the reference

limit values (minimum/threshold values) that must be guarantee at international level are reported. As regards the assessment of comfort, the indicators are grouped into categories according to the number of parameters included, or the specific field of application for which they were proposed.

#### **2. Literature Search**

The literature research started by separately search in the scientific databases "ScienceDirect", "MDPI", "Web of Science" (WOS) and "Google Scholar" the terms related to health and comfort evaluation of the four main environmental factors: "thermal comfort", "thermal stress" "indoor air quality", "indoor air pollution", acoustics comfort", "noise exposure", "visual comfort", "visual fatigue". The summary of the results of this search are reported in Table 1. It is possible to observe that "thermal comfort" and "indoor air quality" are the most studied topics, while "acoustic comfort" is the subject on which it was less published. However, it should be considered that, although the terms "thermal comfort" and "indoor air quality" are the specific terms uniquely used to refer to the relative factor, "visual comfort" and "acoustic comfort" are not always the only terms used. In fact, the term "acoustic comfort" is sometimes replaced by "aural comfort" or "sound quality".


**Table 1.** Results of general literature search in different scientific databases.

Concerning the databases, it can be noted that a higher number of papers was obtained from Google Scholar, and this is also due to the impossibility to set the search option. In Science Direct and Web of Science (WOS) it was also possible to distinguish the paper type and it is was useful to easily identify the review papers.

Among these papers, the most "relevant" and the most "cited" (results sort type) were selected and reviewed by reading title, abstract and keywords. Special attention was paid on the review articles (numbers in brackets in Table 1) because they provided already organized information. Then the bibliographies of such papers were examined, in order to identify possible other interesting researches not present in the previous results. Subsequently, using the references of the consulted papers, a search was carried out aimed at identifying the original papers in which these indicators were presented for the first time.

At the same time, the websites and publications (guidelines) on these issues of the main international organizations such as the World Health Organization (WHO) and the European Agency for Safety and Health and Work (EU-OSHA) were consulted, as well as the European Directives and international standards. Overall, around 300 documents were collected which were the subject of this review article.

#### **3. Health and Comfort Evaluation**

Nowadays, it has become essential to guarantee high standards of occupants' health and comfort in indoor environments (especially in the workplaces) [9,32–35]. To achieve high levels of IEQ, it is necessary to consider that the human perception of the indoor environment is affected by four basic environmental factors: thermal environment, air, acoustics, and lighting. The evaluation of each factor should be different in relation to the purpose of the investigation. Indeed, if it is necessary to consider that two different approach are generally followed for the assessment of the risk for human health and the evaluation of the human comfort.

The assessment of human health risks regards the human exposure to potentially dangerous conditions; its evaluation must include the containment measures and the risk prevention activities that are considered primary interventions. The safety of occupants represents an essential need for all environments. The aspects involved in the occupants' safety must be never overlooked. The evaluation of the risk for human health shall be generally carried out measuring some specific parameters and comparing the obtained values with the threshold values provided by the national or international standards and laws. The comparison (pass/fail) is purely quantitative.

In the ASHRAE TC 1.6 (Terminology), the IEQ is defined as "a perceived indoor experience about the building indoor environment that includes aspects of design, analysis, and operation of energy efficient, healthy, and comfortable buildings" [36]. The effects of problems related to poor IEQ levels on occupant comfort, health, well-being, and productivity were clearly demonstrated by the scientific literature [37–39]. These problems can also have consequences on the lifecycle costs and on the additional energy consumption due to the attempts to compensate for the non-adequate environmental conditions [40].

Although to guarantee a risk exempt environment is a basic need, it is not sufficient to guarantee a high level of well-being that instead can be achieved only considering the comfort perceived by the occupants, too. Figure 1 represents a map for reading the rest of the paper: each of the next sections is dedicated to an environmental factor, and for each of them, the indicators relating to comfort and risk to human health are reported and classified.

**Figure 1.** Main aspects related to the comfort and to the risk for human health for the four IEQ environmental factors.

With regards to comfort and health, in the ASHRAE guideline 10-2016 "Interactions Affecting the Achievement of Acceptable Indoor Environments" [1], the four main environmental factors and the interactions among environmental conditions are treated, highlighting the importance to not limit the analysis to the single environmental factor. Furthermore, "in order to provide an acceptable indoor environment, it is necessary not only that each aspect of the environment be at a satisfactory level but also that the adverse impact of interactions between these aspects is limited" [1]. The environmental factors do not necessarily have equal importance (weight), but in any case, if an environmental factor is rated as least satisfactory, it is also likely to be rated as the most relevant one. It is therefore essential that all aspects involved in the four environmental factors should be considered satisfactory in order not adversely affect the overall satisfaction. In the overall evaluations of the indoor environment, it is therefore important not to limit the assessments to quantitative evaluations only, but also to include qualitative considerations.

#### **4. Thermal Environment**

The thermal environment involves such different environmental aspects as air temperature, surrounding surface temperatures, air speed and humidity. Furthermore, the human perception of thermal comfort depends not only on physical factors, but also on metabolic activity, clothing, and personal preference. As stated by Parsons [41]: "The challenge for every person is to successfully interact with his or her local environment. The human body responds to environmental variables in a dynamic interaction that can lead to death if the response is inappropriate, or if energy levels are beyond survivable limits, and it determines the strain on the body as it uses its resources to maintain an optimum state. In the case of the thermal environment this will determine whether a person is too hot, too cold or in thermal comfort. Air temperature, radiant temperature, humidity, and air movement are the four basic environmental variables that affect human response to thermal environments. Combined with the metabolic heat generated by human activity and clothing worn by a person, they provide the six fundamental factors (sometimes called the six basic parameters as they vary in space and time but fixed representative values are often used in analysis) that define human thermal environments".

#### *4.1. Thermal Stress*

#### 4.1.1. Overview

Thermal stress can severely affect human health, but it can also reduce attention span, increasing the risk of injury. Exposure to inadequate thermal conditions can reduce productivity and tolerance to other environmental risks. Thermal stress represents a relevant factor in many industrial situations, sports events and military scenarios [42]. In the industrial context, when people are exposed to extremely hot or cold environments, thermal discomfort is considered as one of the major causes of dissatisfaction [43]. People who work in severe hot or cold environments may lose the ability to make decisions and/or perform manual operations, and therefore the likelihood that they will behave unsafely increases. When it becomes difficult to control internal body temperature, heat stress occurs.

#### 4.1.2. Guidelines and Legislative Outline

The control of thermal risks in workplaces is not included in any specific EU Directives, although such risks are covered by the general provisions of the Framework Directive (89/391/EEC) [44,45]. In any case, general recommendations on the protection of workers from thermal risks and against thermal hazards are provided in the guidelines of Workplace Safety and Health Services of the occupational national health authorities [46]. All these guidelines suggest that the temperature in the work environments should be adequate for the human body during all the work activities taking into account the physical effort of the workers. It is not sufficient consider only the effects of the temperature, but is necessary to take into consideration also the influence of humidity and ventilation in the perception of the work environment. In fact, in addition to the air temperature, other factors can help to minimize both the heat stress and the thermal discomfort. When it is not convenient to modify the temperature of the entire environment, workers must be protected against too high or too low temperatures by means of collective or individual protection devices [47]. Furthermore, Malchaire et al. [48] developed a strategy to assess the risks of heat disturbance in any work situation. This strategy is based on the three highest phases of the SOBANE strategy: "observation" to improve the thermal conditions of work; "Analysis" to assess the extent of the problem and optimize the choice of solutions; "Expert", when necessary, for an in-depth analysis of the work situation [49,50].

#### 4.1.3. Indicators

The creation of a single indicator that allows the evaluation of the perceived thermal stress in a wide range of environmental conditions and physiological activities required numerous attempts [42]. In the Table 2, according to [27,42,51], the most important thermal stress indicators proposed since 1945 are summarized and divided according to their application in hot or cold environments.

Among the thermal stress assessment indicators for hot environments, the most common is the wet bulb globe temperature (WBGT). The WBGT was introduced by the US Navy within a study on accidents related to heat during military training [52], and it was derived from the "corrected effective temperature" (CET). The WBGT comprises the weighed combination of the dry bulb (dry-bulb) (Ta), the wet bulb (wet-bulb) (Tw), and the black-globe (Tg) temperatures. Even if the WBGT has no physiological correlations, weighted coefficients were obtained empirically. The reference values of the WBGT are reported in Table 3.


**Table 2.** A collection of thermal stress indicators.

Both ISO and ACGIH (American Conference of Governmental Industrial Hygienists) recommend considering the WBGT index as a screening tool, while the PHS (Predicted Heat Strain) approach must be used to investigate more severe working conditions in the heat. Based upon a rational approach, PHS it is the only index allowing the assessment of the work limit duration [87].

Concerning the cold environment, the most used indicator is the required clothing insulation (IREQ). The IREQ model was proposed in 1984 by Holmer [54] for the assessment of thermal stress during work in cold environments. The IREQ combines the effects of different thermo-hygrometric parameters (i.e., air temperature, mean radiant temperature, relative humidity, air velocity) with and metabolic activity and, in order to maintain its thermal equilibrium, it specified the required clothing insulation [88]. The IREQ is created upon rational basis and it is recommended by ISO 11079 [82,89].


**Table 3.** Example of WBGT reference values.

#### *4.2. Thermal Comfort*

#### 4.2.1. Overview

The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) defined thermal comfort as "the condition of the mind in which satisfaction is expressed with the thermal environment" [90]. In addition to environmental conditions, the perception of thermal comfort can be influenced by personal differences in mood, culture and other social factors. In the ASHRAE definition, thermal comfort is not only a state condition, but rather a state of mind (a cognitive process that involves many inputs influenced by physical, physiological and other factors). "Satisfaction with the thermal environment is a complex subjective response to several interacting and less tangible variables" [29]. An absolute thermal comfort condition does not exist; the sensation of thermal comfort occurs when the body temperatures fall in rather narrow intervals, the moisture on the skin is low, and the physiological effort is minimal [29].

#### 4.2.2. Indicators

Nowadays, for the evaluation of the thermal comfort, two main approaches are proposed: the rational or heat balance approach, and the adaptive approach. The first uses data from climate chamber studies to support its theory, the second uses data from field studies of people in buildings.

The heat balance approach is based on the results of the experiments carried out by Fanger in 1970 [91]. Using a steady-state heat transfer model, Fanger studied the response of a sample of 126 Danish student who underwent specific tests in a climate chamber [92]. Then, in order to determine range of comfort temperature in which the occupant feel comfortable, he combines theories on heat balance with the physiology of the thermoregulation. In order to maintain a balance between the heat produced by metabolism and the heat lost from the body, the human body is engaged in physiological processes (e.g., sweating, shivering, regulating blood flow to the skin). Consequently, the condition in which this thermal equilibrium is maintained can be considered the first condition to obtain a neutral thermal sensation [29]. Then, the tests were repeated until they involved a sample of 1296 participants, and a comfort equation was obtained that could predict when the occupants feel thermally neutral.

With the Fanger comfort equation it is possible to calculate all combinations of the environmental variables (air temperature, air humidity, mean radiant temperature and relative air velocity) which will create optimal thermal comfort for any activity level and any clothing insulation [92]. The comfort equation related thermal conditions to the seven-point ASHRAE thermal sensation scale and became known as the predicted mean vote (PMV) indicator. Furthermore, to the PMV, the predicted percentage of dissatisfied (PPD) was added in order to obtain a direct information of the percentage number of people who may experience discomfort. Using the Predicted Mean Vote and the PDD indicators it is possible to assess the thermal sensation perceived by occupants of indoor environments. It is important to note that the PMV should be calculated taking into account the range of application and the limitations, as it is very often improperly used without taking into account all the input variables, or calculated outside its range of validity [93,94].

The adaptive thermal comfort approach is based on the consideration that occupants are passive subjects, but they constantly interacting with the environment and adapting to it [95]. Using this approach, thermal perception is determined in relation to the indoor operating temperature and the outdoor air temperature. The adaptive thermal comfort approach takes into account that there are different factors that affects the thermal perception. These factors can include such different aspects as: demographics (gender, age, economic status), context (building design, building function, season, climate, social conditioning), and cognition (attitude, preference, and expectations) [96]. The adaptation can be divided into three different categories: behavioral, psychological, and physiological.

Behavioral adaptation is the most important factor and it offers the opportunity to adjust the body's heat balance to maintain thermal comfort by changing the clothing levels, opening or closing the windows, and switching on/off the fans. Psychological adaptation describes how habits and expectations can change perceptions of the thermal environment and, more specifically, it refers to the effects of the cognitive, social, and cultural variables. Physiological adaptation or acclimatization is the less relevant of the three aspects in the moderate range of more common indoor environmental conditions [97].

Although the Fanger indicators (PMV and PPD) are the most widely used [15–17], in the literature, other indicators for the evaluation of thermal comfort were proposed, among which some of these were designed to better evaluate the thermal sensation in particular climatic conditions. A collection of the indicators found in the literature with the reference of the scientific publications in which they were proposed, is reported in Table 4. These indicators were classified in relation to the number of parameters necessary for their determination. In particular, they were classified in: "single parameters based" (if directly obtained from one parameter), "double parameters based" (if obtained from the combination of two parameters), and in "multiple parameters based" (if obtained from the combination of three or more parameters).

#### 4.2.3. Current Research Trends

Climate change, the urgency to decarbonize the built environment, and the reduction of energy consumption are still the topics of greatest interest. The increased policy attention on these topics has increased research and development investment. The demand for increasingly high-performance buildings has led to the research of new strategies to mitigate energy consumption and environmental impact. The new challenge is to minimize energy consumption by guaranteeing high housing standards. For this reason, the thermal comfort indicators suitable for evaluating air-conditioned environments and occupant interaction with plant systems are still studied.

The use of adaptive thermal comfort models can be particularly well combined with climate-based software for the dynamic simulation of the buildings behavior that allow to study the variation of internal conditions in relation to the external temperature changes. The comfort simulation methods based on easily accessible numerical simulation tools of the built environment are becoming more popular. A possible critical aspect of this method concerns the risk of moving too far away from the actual behavior of the users, which is very often difficult to predict. Practitioners' confidence in comfort models (design phase) and in engineering calculations, as well as conformity assessments for existing buildings, is widespread and perfectly reasonable. However, subjective surveys of thermal comfort are a superior contribution to knowledge, with more lasting value for the research community than simulated comfort assessments coming from a comfort model [98].


#### **Table 4.** A collection of thermal comfort indicators.


**Table 4.** *Cont.*

Note (\*\*): indicators that require monitoring in pre-established time intervals and the determination of the percentage of time in which a certain parameter falls outside the relative reference value.

#### **5. Indoor Air Quality**

Clean air is considered a basic requirement of human health, well-being, comfort and productivity [139]. Indoor air quality is affected by different factors as the interactions between building materials, building services, location, climate, contaminant sources, and occupancy [140–143]. Indoor air quality can be approached from three points of view: the human, the air of the indoor space, and the sources contributing to indoor air pollution [136].

From the human point of view, indoor air quality is the physical effect of people's exposures to substances in the indoor air. From the indoor air point of view, indoor air quality is often expressed with the ventilation rate or with the concentrations for specific compounds. From the sources point of view, air pollution levels may be higher near specific air pollution sources such as roads; the protection of people from air pollution may require specific measures to maintain the pollution levels below the threshold values indicated in the national and international guidelines [139].

#### *5.1. Indoor Air Pollution*

#### 5.1.1. Overview

Considering that especially in urban areas, people spend long periods of time in indoor environments, the international scientific community deals with air pollution in living environments because indoor pollution can cause side effects ranging from discomfort to serious health consequences. In this regard, illness such as sick building syndrome (SBS) [144,145] and building related illness (BRI) are referred to [145]. It is important to note that a study conducted by the United States Environmental Protection Agency (EPA) in 1998 has estimated that indoor concentrations and indoor exposures are generally 1–5 times, and 10–50 times higher than outdoor ones, respectively [146]. In 2000, the World Health Organization (WHO), with the document "The Right to Healthy Indoor Air", recognized a healthy indoor air as a fundamental human right [147].

#### 5.1.2. Guidelines and Legislative Outline

The WHO, with "the health for all in the 21st century", provided a policy framework for the European region [148]. The target was that, by the year 2015, people in the EU region had to live in a safer physical environment, with contaminants exposure lower than internationally agreed thresholds. This target has consequently resulted in the introduction of legislative measures for the regulation of the surveillance and control of the air quality in both indoor and outdoor environments.

The healthiness of the indoor air is influenced by such different factors as undesirable substances emitted by construction, furniture materials, and human being, as well as ventilation and heating systems if not adequately cleaned. Concerning the indoor air quality, it is important to consider individual exposure to pollutants that can be defined as the air pollutants concentration over time. This depends upon several aspects: the emission rate of a pollutant (μg/s or μg/s per m<sup>2</sup> surface area of source); the ventilation rate of the space in which the pollutants are produced (m3/h or L/s); the pollutants concentration in the ventilation air (ppm or μg/m3).

In Italy, a clear legislative framework for the control and the maintenance of the air quality in the indoor environment does not exist. In the national law 81/2008 [47], the only limit values provided are referred to the concentrations of CO2, radon, and bacteria allowed in the indoor environment during the daily activities. For example, the maximum permissible level of CO2 is equal to 5000 ppm in a time interval of eight working hours.

#### 5.1.3. Air Pollutants

At the international level, the exposure limit values of a wide range of pollutants were defined by the EU and the WHO [44]. Since 1958, when the first report was published, the WHO has been working on the potential adverse effects on health of air quality [149–151]. In the first edition of the WHO air quality guidelines (AQGs) [152], 28 air pollutants (organic and inorganic) were classified, and different

approaches to deal with carcinogenic (i.e., unit risk factors) and non-carcinogenic-health (i.e., LOAEL and protection factors) aspects were used. Furthermore, SO2 and particulate matter (PM) were considered jointly. The second edition of the WHO AQGs was published in 2000 [153], following the evidence of health effects occurring at lower levels of exposure, and it was considered as a starting point for the EU Air Quality Directive and the definition of legally binding limit values [154]. In this AQGs 32 air pollutants are concerned (see Table 5), moreover the assessments for three organic air pollutants (butadiene, polychlorinated biphenyls and polychlorinated dibenzodioxins and dibenzofurans) and a specific section for indoor air pollutants (radon, environmental tobacco smoke, and man-made vitreous fibers) are included.

The latest WHO AQGs "WHO Air Quality Guidelines, Global Update 2005" [155] concerned the policy development and risk reduction application of AQGs, as well as a comprehensive risk assessment for the four classical air pollutants: PM, O3, NO2, and SO2. In addition to numerical guidelines, it also proposed interim targets above the guideline value to promote steady progress in different world regions [155]. Moreover, the WHO has published a series of indoor AQGs on dampness and mould, selected pollutants, and household fuel combustion [156–158]. In Table 6 are shown the limit values of the air pollutants with the latest version of the relative WHO Air quality guidelines (WHO AQGs).




#### **Table 6.** Example of limit values for the air pollutants.

#### *5.2. Good Indoor Air Quality*

#### 5.2.1. Overview

Indoor air quality (IAQ), in terms of comfort, is very important because it affects building occupants and their ability to conduct activities, in addition to creating positive or negative impressions on customers, clients, and other visitors.

When IAQ is not adequate, building owners and managers must devote considerable resources to resolve occupant complaints or deal with long periods of building closure, major repair costs, and expensive legal actions. On the contrary, when IAQ is good, building are more desirable places to work, to learn, to conduct business, and to rent [159,160].

High levels of IAQ are achieved by providing air in which there are low contaminant concentrations and no conditions that can be associated with occupant health or comfort complaints. The current knowledge on the health and comfort impacts of specific contaminants and contaminant mixture in nonindustrial environments, does not allow for the development of a single IAQ metric able to provide a summary measure of IAQ in buildings [159]. Considering the difficulties associated with the air quality assessment in confined environments, to date has not possible to formulate a universally accepted air quality definition. At present, for buildings, the definition of the ASHRAE considers acceptable the indoor air quality when "it does not contain known contaminants in harmful concentrations, as established by the competent authorities, and for which a substantial majority of people exposed (80% or more) does not express dissatisfaction" [161].

#### 5.2.2. Indicators

Regarding the air quality assessment indicators, it is certainly important to mention the 1987 Fanger study [162]. Fanger, similarly to what was previously done for the evaluation of thermal comfort, introduced a subjective indicator called DECIPOL. The DECIPOL is defined as "pollution perceived in the presence of a normal subject (1 olf) in an environment with ventilation equal to 10 L/s of clean air". In the DECIPOL definition, 1 olf represents the main quantity and it is defined as the "amount of bio-effluents emitted from a standard source consisting of a subject that performs sedentary activities in conditions of thermal well-being with a hygienic standard of 0.7 baths/d" [162].

Later, different IAQ indicators were proposed for the evaluation of IAQ in buildings [163–177]. Two different approaches are commonly used to construct IAQ indicators: subjective surveys and field measurements [178].

The IAQ indicators based on subjective surveys include the administration of questionnaires on the perception of IAQ and indoor comfort, (e.g., the ABCD tool proposed in the Netherlands [179]) or the compilation of checklists describing building facilities (e.g., Indoor airPLUS proposed by the USA Environmental Protection Agency [180]).

The IAQ indicators based on field measurements, such as the BILGA index [163,164] and the IAQ Certification [165], are more common, and generally they can be calculated using equations, e.g., the indoor environmental index (IEI) proposed in the USA [176–178]. According to [181], the IAQ indicators can be classified according to used approaches in: "one indicator per single pollutant"; "simple aggregation"; "aggregation according to the sources of pollutants and/or types of pollutants"; "aggregation accounting for the IAQ of the building stock"; "aggregation by simple addition of health impacts". Among the more recent studies, it is possible to find comfort indicators built on the basis of the PMV proposed by Fanger for the thermal comfort. In particular, Zhu and Li [182] tried to connect the objective environment parameters with the subjective comfort perception introducing the indoor air quality indicator PMVIAQ. Such an indicator is defined as the maximum value of the PMV indicators related to three different pollutant concentrations: CO2 (PMVCO2), PM10 (PMVPM10), and the formaldehyde (PMVHCHO).

The main indoor air quality indicators proposed in literature are collected in Table 7.



\* One index per single indicators are commonly obtained with the ratio between the concentration of the pollutants and the relative exposure limit value.

#### 5.2.3. Current Research Trends

Despite the existing air quality indicators, the lack of metrics which quantitatively describe the IAQ can be regarded as one of the most relevant complications for the achievement of the integration of energy and IAQ strategies in indoor environment design. Such an indicator could allow the analysis and comparisons of different plans for achieving high IAQ levels, high energy performance, and low greenhouse gas emissions [183]. Recently, the International Energy Agency (IEA) in agreement with the Energy in Buildings and Communities (EBC) defined a project (IAE EBC annex 68) to provide a guideline for the design and control strategy of high energy efficiency residential buildings. One of the first steps of this project is to define an indicator that will have to include the additional energy consumption necessary to improve IAQ (compared to standard practices), e.g., an increased consumption induced by higher air change rates [181,183].

#### **6. Acoustical Environment**

People are continuously exposed to noise, even when they sleep. Consequently, for workers, whether alone in a private office or among a large number of colleagues in an industrial setting, a complete absence of noise never occurs. A good acoustical environment should not involve any physical, physiological or psychological effects on the human body that could negatively affect health. Furthermore, the acoustical environment should allow a person to be in the most suitable state of mind for a specific activity [184]. On the contrary, exposure to noise can affect quality of life and, in the worst case, can lead to health problems. A workplace with good acoustics allows for confidential conversations between collaborators without affecting those engaged in individual work. The internal environment must protect from excessive noise pollution from internal and external sources and encourage the performance of the planned activities, specifically in all those environments where verbal communication and correct listening take priority (i.e., schools, conference rooms, etc.).

#### *6.1. Noise Exposure*

#### 6.1.1. Overview

The effects that caused by excessive noise exposure are now well known and they can even lead to hearing loss (e.g., hypoacusis). Noise exposure can also cause effects on other organs such as the cardiovascular, the endocrine, and the central nervous systems, and it involves different effects such as fatigue, interference with sleep and rest, and reduction of work performance. Another possible effect on safety that cannot be neglected is that noise can produce masking effects that can disturbs the verbal communication with other persons and impedes the correct perception of warning signals. Such an effect can increase the risk of accidents at work. However, it is not acceptable either a completely silent environment because it can cause a sense of estrangement [185]. For these reasons, human exposure to noise should be evaluated in the field of safety in the workplace, but also in the domestic and private sphere, using indicators that take into account noise exposure even during the rest hours.

#### 6.1.2. Guidelines and Legislative Outline

The legislative framework in the field of the human exposure to noise in work environments is very clear today. At the international level, the recommendations are given in the Physical Agents (Noise) Directives [186], where the minimum requirements for the protection of workers from risks arising from exposure to noise are provided. Such a directive was implemented by the European Union member states, including Italy with the law 81/2008 [47].

The assessment procedure of noise human exposure is indicated in the Technical Standard ISO 9612 [187] and is composed by the following phases: work analysis, selection of measurement strategy, measurements, error handling and uncertainty evaluations, equivalent exposure levels calculation, and presentation of results. In these procedures the choice of the most appropriate measurement strategy is extremely relevant in order to actually detect the effective human exposure and to estimate the results uncertainty. The indicators for this type of assessment are the daily exposure and the peak noise. Their action levels and limit values are given in [186] to ensure that appropriate actions are taken to guarantee the protection of workers. Personal hearing protection must be used when other controls cannot adequately reduce the noise exposures [188]. Hearing protection must be selected to reduce noise exposure without isolating the worker (overprotection), which may compromise safety.

In the Physical Agents (Noise) Directives [186] two exposure action values (EAV) and an exposure limit value are defined (Table 8). The two EAVs are the "upper exposure actions", and the "lower exposure actions" values. To the first correspond the noise exposure level, above which the employer must require the use of the personal protective equipment, and to the second correspond the noise exposure level above which the employer must provide the workers with the personal protective equipment (but it is not required to oblige their use). The exposure limit value represents the noise exposure level which must not be exceeded.

#### 6.1.3. Indicators

It is important to note that sometimes, despite the continuous equivalent levels being lower than the limit values, exposure for a long time to noise characterized by particular frequency emissions (for example continuous noise at low frequencies) can still cause health problems. In this case, there is no risk of hearing loss, but rather a series of side effects (extra-auditory effects) such as headaches, dizziness, problems with the gastrointestinal system, increased respiratory rate, etc. The psychosocial effects are reflected on the modification of interpersonal relationships and on social relations. In addition to the Physical Agents Directives [186], international level guidelines (e.g., [189,190]) were published, in which indicators and the relative reference values, above which the first effects of noise on human health can begin, are provided (Table 8).


#### **Table 8.** A collection of noise exposure indicators.

Note (1): referred to outside exposure levels. Note (2): Levels above which the first health effects start to occur. Note (3): Limit values related to the lower exposure action values, the upper exposure action values, and exposure limit values, respectively.

#### *6.2. Acoustic Comfort*

#### 6.2.1. Overview

The term "acoustic comfort" is not commonly used; a good acoustic environment is mainly associated with preventing the occurrence of discomfort. An acoustically comfortable building is able to isolate occupants from internal and external noise and at the same time to guarantee an environment acoustically appropriate for the activities carried out within it [191]. It can be defined as "a state of contentment with acoustic conditions" [192]. The acoustic sensation does not depend only on the sound pressure level but also on the sound frequency composition. The acoustic quality of an environment therefore depends on how the sound is propagated inside it (how the sound is absorbed and reflected by the surfaces), how the envelope structure is isolated from external noise, and how the sound coming from inside sources is treated. The acoustic comfort evaluations in workplaces are often undervalued, but especially in environments like offices where annoyance due to the noise can often cause mental stress and concentration loss, it is very important [44]. Very often, the acoustic comfort is considered adequate when the building envelope is sufficiently isolated from the noise coming from outside or adjacent environments (e.g., residential buildings). However, in many other cases, noise insulation is not sufficient to consider an environment acoustically comfortable and more detailed parameters should be used to obtain important information on the correct perception of sounds.

#### 6.2.2. Indicators

In the literature, several indicators for the evaluation of the acoustic comfort in indoor environment are introduced. Such indicators can be divided into three wide groups: sound pressure levels, architectural acoustics, and building acoustics (Table 9).

The first group is composed of indicators related to the sound pressure levels established in an environment due to noise produced by a source located outside or inside the analyzed room.

The second group is composed of indicators which describe the acoustic transient and the quality of sound perception within the analyzed environment.

The third group is composed of indicators that do not describe directly the sound perception, but describe the sound insulation of buildings from noise coming from outside and adjacent environments and the levels of noise produced by the building services.

Among the sound pressure levels indicators, the most widely used is the A-weighted equivalent sound pressure level LeqA. This takes into account the duration, the variation in time, and the frequency composition of the sound. [193]. However, this indicator may not always be the most suitable, while other more specific indicators can be preferable. Among the architectural acoustic indicators, the most common is the reverberation time (RT60), although in literature many other indicators for detailed evaluations of the acoustic quality of rooms are present [194].

#### 6.2.3. Current Research Trends

The evaluation of acoustic quality, especially in schools, is a very topical subject. One of the problems related to the evaluation of acoustic quality is that most indicators are obtained from field measurements or advanced simulations, and only a few indicators can be calculated in the design phase with simplified formulas. For this reason, topics of research concern the determination of equations based on an empirical basis (generally obtained using field measurement results) to correlate the indicators with the reverberation time, which can be calculated with simple and well-known equations. For example, in Italy, for the educational buildings, the speech transmission index (STI) must be verified during the design stage, but the calculus of STI requires field measurements or advanced simulations that are incompatible with the level of detail of the preliminary stage [195,196].


**Table 9.** A collection of indicators for the evaluation of the acoustic comfort in indoor environment.

A second current research topic concerns the proposal of assessment methods of the overall acoustic quality that allow to evaluate the existing environments and to identify all the critical aspects that need improvements. These methods are generally based on Multicriteria analysis in order to combine the results of different indicators and obtain an overall rating [236,237]. In acoustics, only single parameter-based indicators exist, and for this reason, the multi-criteria approach is very interesting. This represents a potential overall assessment tool taking into account more indicators simultaneously. Until now, in fact, in acoustics, the individual indicators are determined and evaluated separately, and the combination of results is decided by the evaluator on the basis of subjective personal knowledge and sensitivity.

#### **7. Visual Environment**

Lighting has significant relevance on comfort and human life [238], with direct and indirect (non-visual) effects on the human body. The direct effects are related to visual performance (visual task activity) and visual comfort; the indirect effects are related to the possible consequences on safety and health. Traditionally, lighting assessment is limited to the assessment of illuminance in the task areas, however, there is a growing awareness of the effects of light on both health and the quality of human life. This is confirmed by new certification systems as the building certification system, developed by the International WELL Building Institute with the aim of "measure, monitor, and certify the performance of building features that impact health and well-being" [239].

#### *7.1. Non-Adequate Light Exposure*

#### 7.1.1. Overview

Properly lighting is a basic need for visual performance and safety. Non-adequate lighting systems can cause problems with visual fatigue, as well as causing errors or possible accidents. In work environment special attention is focused on the video display terminal (VDT) workstations: electric lighting must guarantee adequate levels of illuminance for the reading and writing activities, but they should also favor a satisfactory contrast of luminance between the main object of view (the display) and the background in order to reduce visual fatigue [240,241]. For the video-terminal workstation, the asthenopic complaints are considered signs of visual discomfort. The most common visual discomfort markers are: red eyes, burning eyes, double vision, eye fatigue, blurred vision, headache, etc. [241–243]. Such effects together with inadequate lighting conditions can be related to the computer vision syndrome (CVS) [244]. At the end of 1980s, Bergqvist [245] revealed that eye discomfort and hand/wrist problems were associated with the work at the VDT workstation. Subsequently, several researches have further investigated the consequences of incorrect postures during VDT use [240–254] and the psychological effects of computer use [248]. Exposure to light is studied in relation to its photobiological effects, in particular the consequences of artificial optical radiations (AOR) emitted by light sources are evaluated. If in the past general lighting was considered risk exempt, recently, the advent of new LED sources has rekindled interest in this field, especially in the blue light hazard assessment [255]. In any case, the assessment of exposure to artificial optical radiation (not only produced by general lighting sources) is part of the physical agent, and it is included in the workers risk assessment procedures.

#### 7.1.2. Guidelines and Legislative Outline

At the international level, Directive 89/654/EEC [256] includes an indication about lighting, and provides the requirements to guarantee health and safety in the workplaces. In [257] it is indicated that: "the workplace must have sufficient daylight and be equipped with devices that allow artificial lighting adequate to ensure the safety, health and welfare of workers". This directive was included in the Italian Law 81/2008 [47], where only rather general instructions (without specific limit values) are provided.

Lighting not only affects visual performance and visual comfort, but also human health with so called "non-visual effects". In particular, it is necessary to evaluate human exposure to artificial optical radiation (AOR) emitted by light sources. Optical radiation regards the electromagnetic radiation in the wavelength range between 100 nm and 1 mm. They are divided in: "ultraviolet" (UV), "infrared" (IR), and "visible light" (VIS) or more simply "light" radiation [258]. Biological effects of AOR affect the skin and eyes but systemic effects may also occur [259,260]. Regarding the optical radiation, the Directive 2006/25/CE [257] represents the basic guideline. This directive is adopted in the Italian Law 81/2008 [47], and for a series of wavelength ranges specifies indicators and limit values for the evaluation of the AOR workers exposure. On these issues, important documents are those published by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) [261,262], and the recent international standards [263,264].

#### 7.1.3. Indicators

As indicated in the previous section, the national and international legislation does not provide limit values, and for this reason, the evaluations are generally qualitative and it is necessary to use the same indicators used for the evaluation of visual comfort (see Section 7.2).

The only field in which specific indicators are provided by the legislation is the artificial optical radiation (AOR). The AOR are included in the physical agents and it is required the evaluation for the workers' risk assessment. In Table 10, for each wavelength range, the exposure limit values, the type of hazard, and the action spectrum that must be used are indicated.


**Table 10.** Example of exposure limit values related to the artificial optical radiation.

#### *7.2. Visual Comfort*

#### 7.2.1. Overview

In EN 12665, visual comfort is defined as "a subjective condition of visual well-being induced by the visual environment" [265]. Lighting, daylighting, and the use of colors have a significant impact on the perception of the environment, and can affect both physical and mental well-being. Visual comfort was commonly studied through the evaluation of some factors that characterize the relationship between human needs and the enlightened environment [30].

In inadequate lighting conditions, visual discomfort may not be immediately perceived due to the effect of adaptation of the visual apparatus, but may affect work performance or lead to visual fatigue.

#### 7.2.2. Indicators

The indicators for the visual comfort evaluation can be separated in four groups: amount of light, color rendering, daylight availability, and glare. For each group, a collection of indicators is proposed as reported in Table 11.


**Table 11.** A collection of visual comfort indicators.

Amount of light concerns the quantitative lighting parameters commonly used for the lighting design. They are generally used to describe the performance of electric lighting systems, or in other words, if the lighting systems allow creating sufficient light conditions. In this group, the most used indicator is the illuminance over the task areas.

Color rendition concerns the property of a light source to return the colors correctly. In this group the most used indicator is the color rendering index (CRI). Nevertheless, with the diffusion of LED sources, this indicator is no longer considered the most reliable. New indicators are now under investigation, as for example the color quality scale [271] and the memory color quality metric [275].

Daylight availability concerns the amount of daylight that can be exploited in an environment. This group includes quantitative indicators aimed at assessing the penetration of daylight into the environment, while some other indicators make it possible to carry out assessments on extended time profiles and to estimate the reduction in yearly electricity consumption due to the lower use of electric lighting. In this group, the most used indicator is the daylight factor (DF). However the improvement of simulation software allows to evaluate the daylight availability day by day in real condition (Climate Based daylight modelling software). With the development of such new software the creation of new indicators (e.g., useful daylight illuminance, daylight autonomy, etc.) was possible. These new indicators have great potentialities, because they allow also for optimizing the use of building automation control systems (BACS) and their use is rapidly becoming more and more common.

Glare concerns the conditions of vision in which, caused by an unsuitable luminance distribution, there is discomfort, annoyance or a reduction in visual performance and visibility. In this group, the most used indicator is the unified glare rating (UGR); however, in the literature, many indicators for the evaluation of glare from different sources were proposed.

In lighting, as in acoustics, overall indicators do not exist, and visual comfort must be evaluated as a synergic combination of the results of more indicators properly chosen for the analyzed environment. In these evaluations, at least one indicator is generally used for each category, while the benchmark values depend on the specific visual tasks carried out.

#### 7.2.3. Current Research Trends

The discovery of new photoreceptors inside the human vision system able to influence the human body physiological functions has opened a new branch [297]. These new photoreceptors, that are called intrinsically photoreceptive retinal ganglion cells (ipRGCs) serve no visual function [298].

It is currently recognized that exposure to insufficient or inappropriate lighting scenarios can bother the standard human rhythms, which can have negative consequences for performance but also for health [299]. The development of new circadian metrics is based on scientific information and expert judgments related to the duration of the exposures, time of the day, intensity, light spectrum, and history of light exposures. [300]. In order to evaluate the potential effects of the light sources on the circadian rhythms, it was necessary to quantify the light exposure in biologically meaningful units [301–307]. Until now, a consensus on the appropriate minimum light exposure threshold to ensure effective circadian stimulus in buildings has not yet obtained. The WELL Building Standard's "Circadian Lighting Design" implements a minimum threshold of 250 Equivalent Melanopic Lux (EML), which must be available for at least 4 h each day. Such requirement can be met with daylight, artificial light (exclusively), or a combination of both sources [239,298,306]. Figueiro et al. [308] recommend exposure to a "circadian stimulus (CS) of 0.3 or greater at the eye for at least 1 h in the early part of the day (equivalent to 180 lux, D65)".

In any case, the relationships between spectral distribution, duration, timing, and intensity of light exposure for optimal circadian health should be further clarified by the research community.

#### **8. Conclusive Remarks**

The main aim of this review is to create a comprehensive framework on the indicators for the health and comfort evaluations in indoor environments that is missing today in the scientific literature. Indeed, until now, only review papers that collect indicators of single environmental factors were published.

Occupants' health and comfort represent two basic needs in indoor environment and, although their assessments are characterized by two very different approaches, in some cases, the selection of the proper indicators is not always so obvious. In fact, for the thermal environment, indoor air quality, and lighting, selection is not so obvious and some indicators are commonly proposed for both health risk and comfort evaluation.

Concerning the thermal environment, in the literature there are numerous indicators but the distinction between comfort and thermal stress indicators is not always clear. This means that, in this field, the boundary between health and comfort is particularly subtle and it is not always easy to distinguish the two approaches. Among the thermal indicators, many can be considered "overall" indicators because they are based on two or more parameters and provide direct information about the human thermal perception. In this paper, 35 heat stress indicators and 46 thermal comfort indicators were collected. Despite the large number of indicators, the most used are historically the PMV for the evaluation of thermal comfort, and the WBGT for the evaluation of thermal stress. Today, the use of adaptive comfort models combined with software simulations that can implement climate-based model are much more frequent, however there are still doubts about how these systems can estimate the actual behavior of the users (strength of the post-occupancy evaluation methods).

Concerning the air quality, few indicators were proposed, however, it is very common to evaluate the single air pollutant concentration and compare the results with threshold limit values more or less restrictive depending on the type of evaluation (i.e., minimum safety evaluation or high levels of air quality). Although the indoor environments are often characterized by pollutants concentrations higher than the external environments (where detailed monitoring takes place), only a small number of indoor pollutants can be commonly measured. In this review, nine indicators for the air quality assessment and the limit values of 17 air pollutants are reported. Given the difficulty of measuring all the indoor air pollutants and linking them to human perception, overall indicators similar to those used for the assessment of thermal comfort, were recently proposed. Such indicators are based on subjective evaluations, but at an international level, it is not yet a consensus on which indicator is most correct to use.

Concerning acoustics, the separation between health and comfort is more evident, because human exposure to noise in work environment is clearly defined in the physical agent risk assessment directive. However, it is important to observe that there are many cases where, despite compliance with the limit values, the noise exposure can lead to a series of extra-hearing side effects that must be analyzed in relation to other typical aspects of work-related stress. In addition, people are continuously exposed to noise, even when they sleep; therefore, also the noise level exposures during the whole day and/or the night-time are relevant. Regarding acoustic comfort, 39 indicators classified into three different groups (sound pressure levels, architectural acoustics, and building acoustics) are collected. Among these, overall indicators based on more parameters do not exist, so it is not possible to directly obtain information about acoustic comfort perception. For this reason, for a complete acoustic comfort assessment, it is necessary to use sets of indicators suitably chosen according to the type of environment analyzed.

Concerning lighting, it must be considered that visual and non-visual effects are related to the use of lighting systems. The visual effects are directly related to the visual performance and they are the first analyzed when the lighting of an indoor environment is evaluated. The non-visual effects should be considered because they can affect the occupants health and well-being. For the assessment of occupants' health, the literature focused on two aspects: the asthenopic effects due to the use of video terminal equipment and the exposure to artificial optical radiation. For the evaluation of the asthenopic effects, there are no limit values or specific indicators, but qualitative assessments are necessary. Artificial optical radiation is included in the physical agents assessment and characterized by a series of indicators according to the different wavelength ranges. For the assessment of visual comfort, 37 indicators are identified and divided into 4 groups (amount of light, glare, color rendition, daylight availability). In lighting, especially for the evaluation of the daylight availability, a rapid

evolution connected to the development of software that allows simulation in real climate conditions is under investigation. New daylight indicators allow to combine the evaluation of lighting with the related lighting systems energy consumption, and to estimate the energy consumption reduction due to the optimization of daylight availability.

The growing interest in the IEQ, which is also reflected in the launch of new building certification systems, has led to a growing need for new overall indicators, sets of indicators, or standardized assessment procedures approved at international level. These indicators can allow to evaluate the human perception of the single environmental factor, as well as to carry out overall evaluations in which the four environmental factors are simultaneously taken into consideration. In this paper, the authors have tried to collect and organize a wide range of indicators for assessing the health and comfort of indoor environments. These indicators are not often clearly identified in the evaluation processes, but the interest in these aspects is increasing and clear information to allow the selection of the most appropriate indicators is necessary.

The present review paper represents the state of the art of the indicators of indoor environmental factors. It should be pointed out that the overall assessments of IEQ could include a selection of the indicators proposed in the literature or the proposal of new indicators that take into account several factors simultaneously. In any case, as suggested in [1], it is necessary to verify that all the aspects analyzed reach satisfactory levels, but also that the adverse impact of the interaction between these aspects is as limited as possible. In any case, when global assessments are carried out, it must be taken into consideration that the global acceptance of an environment does not depend exclusively on the achievement of adequate levels of comfort related to the four environmental factors. This is also influenced by other aspects, such as the level of expectation, the adaptation capacity, personal subjective preferences, as well as the psychological and physiological conditions of the occupants.

**Author Contributions:** All the authors contributed in equal parts to the research activity and to the paper writing. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## *Article* **A Comparative Study on Cooling Period Thermal Comfort Assessment in Modern Open O**ffi**ce Landscape in Estonia**

#### **Martin Kiil 1,\*, Raimo Simson 1, Martin Thalfeldt <sup>1</sup> and Jarek Kurnitski 1,2**


Received: 22 December 2019; Accepted: 19 January 2020; Published: 23 January 2020

**Abstract:** Local thermal comfort and draught rate has been studied widely. There has been more meaningful research performed in controlled boundary condition situations than in actual work environments involving occupants. Thermal comfort conditions in office buildings in Estonia have been barely investigated in the past. In this paper, the results of thermal comfort and draught rate assessment in five office buildings in Tallinn are presented and discussed. Studied office landscapes vary in heating, ventilation and cooling system parameters, room units, and elements. All sample buildings were less than six years old, equipped with dedicated outdoor air ventilation system and room conditioning units. The on-site measurements consisted of thermal comfort and draught rate assessment with indoor climate questionnaire. The purpose of the survey is to assess the correspondence between heating, ventilation and cooling system design, and the actual situation. Results show, whether and in what extent the standard-based criteria for thermal comfort is suitable for actual usage of the occupants. Preferring one room conditioning unit type or system may not guarantee better thermal environment without draught. Although some heating, ventilation and cooling systems observed in this study should create the prerequisites for ensuring more comfort, results show that this is not the case for all buildings in this study.

**Keywords:** thermal comfort; draught; cooling period; open office

#### **1. Introduction**

Modern low energy office buildings require energy efficient heating, ventilation, and air conditioning (HVAC) systems which can provide comfortable and healthy indoor environment. In temperate climate countries, mechanical ventilation and active cooling systems are common practice in such buildings. However, mechanical HVAC systems do not always provide satisfactory thermal conditions [1]. It is important to properly apply control strategies, design and install room cooling units and ventilation supply air elements, as well as to operate and maintain the systems to provide comfortable indoor climate without temperature fluctuations and draught risk in the cooling season [2–8]. Office plans, in terms of occupant positions and density, can be very different from initial design and vary significantly, resulting in changing conditions and dynamic settings which makes it difficult to design the systems adequately to ensure stable thermal environment. Open office layout design is used commonly in most office buildings mainly to allow flexibility in workspaces allocation [9]. This creates a difficult task for HVAC systems design, requiring careful planning to assure adequate conditions in the occupied zone in different layout cases.

As occupant satisfaction with thermal environment is dependent on many factors, such as gender, age, health, activity, mood, and other physiological and psychological factors, assessing thermal comfort

(TC) based on temperature and air movement measurements is usually not sufficient for adequate estimation [6,10–13]. Thus, evaluation by questioning the occupants is usually also needed to specify the problems and get a comprehensive overview of the TC situation. Studies on office workers thermal sensation have shown that the predicted TC and actual sensation can differ significantly [12,14,15]. For example, gender specific analysis indicates higher dissatisfaction rates for female occupants [11,16–19]. Recent research has widely focused on individual perception of TC [20–22], developed methods to analyze the preferences for TC using machine learning algorithms [23,24] and adapt systems to provide preferable personal comfort by implementing Personalized Comfort Systems [20–22,25]. Utilization of such systems in buildings requires paradigm shifts in occupant interaction with HVAC systems as well as system design practices, integration of advanced controls and information technologies solutions [26,27].

In addition to the individual preferences and system specific aspects influencing thermal comfort (TC), there are many building related design factors that can affect the performance of HVAC systems and in turn influence the thermal environment. Of these factors, façade design, namely window sizes, layout, and glazing parameters, can have large impact on cooling load as well as radiant temperature asymmetry and thus major influence on the overall thermal conditions in the office [28,29]. Thalfeldt et al. [28] showed the importance of façade design by analyzing the effect on office buildings energy efficiency and cooling load in cold climate countries. Window-to-wall ratio (*WWR*) of 0.25 was found optimal for triple glazing window solutions. Larger glazing results in higher cooling loads and increase the need for larger room cooling units, higher cooled airflow rates or lower supply air temperatures to maintain the room temperature. The latter factors also increase the risk of draught in occupied spaces. In several studies, draught rate (*DR*) has been identified as the main cause of discomfort even if other thermal environment factors are at satisfactory levels [6,15,29,30].

Depending mainly on the cooling load, cooling plant solution, and interior design, different water based room cooling solutions are used in offices, which can be classified by supply water temperature as low temperature room cooling units e.g., fan coil units and high temperature units, such as thermally active building systems (TABS), passive cooling beams, or active cooling beams, combined with ventilation supply air terminals [31,32]. In low energy buildings, high temperature cooling is usually preferred to achieve higher energy efficiency for cooled water production by cooling plants [32]. The performance of these systems is extensively analyzed in various recent studies. Most of the research is based on either computer simulations, mainly computational fluid dynamics (CFD) studies or studies conducted in controlled laboratory environments [33–47]. The research in real office settings is mainly focused on buildings located in warm and hot climate countries, dominated by cooling need [48–51]. To the knowledge of the authors, only few extensive studies have been carried out in cold and temperate climates and in low energy buildings. In Germany, Pfafferott, Herkel, Kalz, and Zeuschner [14] have conducted research on summertime TC in 12 low energy office buildings which are passively cooled with local heat sink based TABS. Results showed, that 41% of occupants were dissatisfied with thermal environment in summer, but assessment, according to the standard CEN EN 15251 [52], showed measured indoor temperature-based classification relative to the indoor climate category I (highest) and II, indicating a gap between perceived and assessed TC conditions and the need for more detailed comfort assessment. Hens [15] investigated TC in two office buildings in Belgium cooled with active chilled beams and air-cooling systems. He found that the Fanger [53] predicted mean vote (*PMV*)/predicted percentage dissatisfied (*PPD*) curve underestimated the actual number of dissatisfied occupants and that standards should not be considered as absolute references. It was also concluded that one should be very careful when interpreting the results of TC studies.

The theoretical knowledge involved or access to expert engineers during office building design is feasible and implemented in practice in Estonia. However, the volatile quality of different parts of the design, building phase simplifications with budget cuts and the contradiction between the initial task and the actual situation leads to a risk of an outcome failure regarding TC for the occupant. In the Estonian construction market, great emphasis is given on diplomas, professional certificates,

software for both building information modeling (BIM), and product selection programs integrating BIM solutions in the building process. In reality, during the construction process the HVAC designer and after the warranty period, the constructor retreat. Therefore, in a short period of time during the design a huge effort is invested in the project definition, while after the realization phase much of the expert advice is ignored. Otherwise, complaints regarding draught or room temperature were not topical issues. In Estonia as well, in-depth research on cooling season TC and occupant satisfaction is practically non-existent, a few studies in office buildings have been conducted with the main focus on heating season performance and mostly aimed towards energy efficiency analysis. The conducted studies indicate problems and dissatisfaction with thermal environment but lack the detail to specify the causes and details of occupants' thermal conditions and HVAC systems performance in terms of room equipment.

Regardless of the design and performance of the HVAC systems installed in actual open offices, the hypothesis of this study proposes that high proportion of occupants are dissatisfied with the TC conditions. The goal of this study is to determine, whether the thermal sensation dissatisfaction of the occupants in modern office spaces is verifiable and in accordance with the valid standard criteria. This paper aims to fill the gap of summer TC assessment by extensive field studies and thorough occupant survey in modern office buildings in Estonia, a temperate climate country. We have investigated four recently constructed and one reconstructed office buildings with open plan office layouts designed with different ventilation and cooling solutions, including mixing and displacement ventilation, TABS, radiant cooling panels, fan coil units, and active cooling beams. The on-site measurements conducted in the offices consist of high resolution and accuracy temperature and air velocity measurements with *DR* and TC calculations, which are described in the following chapter.

#### **2. Methods**

The flow chart of research methodology is shown in Figure 1. Section of methods is divided between description of reference objects, measurement set-up and equipment specifications, data analysis, and indoor climate questionnaire (ICQ). We used standard-based [52,54,55] methods in this study to measure and calculate TC parameters and to perform an online ICQ survey. The TC measuring probe and tripod mobile and flexible kit set [56] we used was designed for research and development purposes.

**Figure 1.** Flow chart of research methodology.

#### *2.1. Reference Objects*

General information regarding reference objects are provided in Table 1. The scope and range of measurement points with main building envelope characteristics, such as thermal transmittance for main surfaces, such as walls, windows, floors on ground and roofs are listed with specific heat loss of external envelopes and window-to-wall ratios.



In Buildings A, B, C, and D, measurements were also taken on the highest floor and in Buildings B and D on the lowest floor. The temperature of slabs was considered to be close to *ti* and therefore the impact on operative temperature was not accounted for, as heat transmission through the building envelope in such low energy buildings is negligible compared to the heat gains through glazed surfaces and have little effect on TC. A variety of HVAC systems was involved in measurements zones (Table 2) including new and innovative solutions in the Estonian construction market.

**Table 2.** Heating, ventilation, and air conditioning room design solutions of reference objects.


Ø: diameter of air diffuser connection duct.

The buildings involved in this study were chosen from a range of modern office spaces in Tallinn. First criteria for reference objects was the correspondence with the Estonian energy efficiency regulations, which were first set in 2007 [57]. This created the prerequisites for new buildings and HVAC systems criteria, such as envelope related parameters, such as air tightness, external wall insulation thickness, window glazing solutions, and HVAC system parameters, e.g., effectiveness of room units and energy sources, heat recovery effectiveness, specific fan power of ventilation units etc. Buildings A and B have high temperature heating systems and district heating. Building B is using low temperature heating and a ground source heat pump, Building D has high temperature heating water produced and a gas boiler and electrical heating convectors are installed in Building E. All of the studied buildings are equipped with dedicated outdoor air ventilation systems with heat recovery. Ventilation air distribution methods were classified as mixing ventilation, except for Building B, where supply air systems were built in a way to support displacement ventilation method. Buildings A and C were using active chilled beams for supply air distribution. Buildings A, C, and D are built with chillers to supply the cooling system. In all Buildings, except for E, high temperature cooling is used in room conditioning units as supply air is dehumidified in the air handling units. Multi-split fan coil units with refrigerant without the option of heating function were in operation in Building E. Room conditioning units in Buildings C and D, including the Building B with thermally active buildings system, operated both for heating and cooling purposes.

#### *2.2. Measurement Equipment*

Experimental measurements in this study were carried out with a TC measurement system Dantec Dynamics ComfortSense [56]. The system is designed for high quality multi-point measurements of *va*, *ti*, *RH,* and *to*. The set is equipped with software, what allows easy setup for measuring sequence and positions giving researchers a comprehensive overview if the measured data. Measurement equipment probe data is described in Table 3.



The set is mounted on a tripod including five draft probes, one humidity and one *to* probe. For a sitting position, ISO standard [55] recommends measuring heights for ankle level 0.1 m, abdomen level 0.6 m, and head level 1.1 m. Conformably to Fanger and Christensen [6], mean *va* and standard deviation at three heights around the sitting occupant body were measured according to heights shown on Figure 2. RH probe was set at 1.0 m as a fixed height for measuring has not been fixed for measurements. The *to* probe was mounted with the angle of 30◦ at the height of 0.6 m as the abdomen level of a sitting person [55].

**Figure 2.** Recommended air velocity probe heights behind the feet, elbow, and neck for a sitting person (**a**); Recommended operative temperature probe person's angle factor to their surroundings (**b**) [58].

Probes were connected with 54N90 ComfortSense main frame [56], using 7 channels of 16. Main frame was in turn connected with laptop computer where the measurement data was stored using ComfortSense software version 4, (Dantec Dynamics A/S, Skovlunde, Denmark) [56]. Measurement period of 180 seconds as the least time recommended [59] was used.

#### *2.3. Data Analysis*

Measurement data, including *ti*, *va*, *RH*, and *to* was recorded with the sampling rate 20 Hz with ComfortSense [56] and processed in Microsoft Excel. TC parameters are calculated for each measurement positions with equations for *Tu*, *DR*, *PMV*, and *PPD* followed. To assess *DR*, the fluctuation rate of *va* is described as *Tu*, which is calculated by [59]

$$Tu = \frac{SD}{v\_{\text{fl}}} \times 100 \ (\%), \tag{1}$$

where *SD* is standard deviation of measured local mean *va* (m/s) for one measurement. With *ti*, *va*, and *Tu*, the percentage of people predicted to be dissatisfied because of draught may be calculated as [60]

$$DR = \left(34 - t\_i\right) \times \left(v\_{\rm u} - 0.05\right)^{0.62} \times \left(0.37 \times v\_{\rm u} \times T\nu + 3.14\right) \text{(\%)}\tag{2}$$

To predict the mean value of the subjective ratings of a group of people in a given environment, *PMV* index is used. Consisting of a set of parameters with sub-formulas, the *PMV* equation is given by [60]

$$PMV = \left[0.303 \times \exp(-0.036 \times M) + 0.028\right] \times \left[\left(M - W\right) - H\_d - E\_c - C\_{\text{res}} - E\_{\text{res}}\right] \tag{3}$$

The *PMV* index in Equation (3) was calculated using Equations (4)–(11). In the equations provided, *M* (W/m2) is metabolic rate and *W* (W/m2) is the effective mechanical power. Assumption of metabolic rate 1.2 met for sedentary activity for summer season provided in EN 16798-1 was used. Sedentary activity does not suppose producing effective mechanical power, therefore 0 (W/m2) was used in analysis. The next symbol *Hd* in Equation (3) represents dry heat loss, which is found as

$$H\_d = \frac{(mt\_{sk} - t\_{cl})}{I\_{cl}} \Big(\text{W/m}^2\text{)},\tag{4}$$

where *mtsk* is mean skin temperature [◦C] and in Equation (4), *tcl* is expressed using to and calculated through iterative process, by

$$t\_{cl} = 35.7 - 0.028 \times (M - W) - I\_{cl} \times \left[3.96 \times 10^{-8} \times f\_{cl} \times \left[ (t\_{cl} + 273)^4 - (t\_0 + 273)^4 \right] + I\_{cl} \times f\_{cl} \times h\_c \times (t\_{cl} - t\_0) \right] \text{ (\ $/\$ )} \quad \text{(\ $/\$ )}$$

In Equation (5), *Icl* [(m2 <sup>×</sup> K)/W] is the clothing insulation, *fcl* is the clothing surface area factor, *var* (m/s) is the relative air velocity, *hc* [W/(m<sup>2</sup> <sup>×</sup> K)] is the convective heat transfer coefficient, and *tcl* ( ◦C) is the

clothing surface temperature. Clothing unit 0.5 clo for summer season provided in EN 16798-1 was used in calculations. Equation (5) includes *hc*, which is given as

$$\begin{cases} h\_c = 2.38 \times |t\_{cl} - t\_l|^{0.25} & \text{for} \quad 2.38 \times |t\_{cl} - t\_l|^{0.25} > 12.1 \times \sqrt{v\_{ar}} \text{ and} \\ 12.1 \times \sqrt{v\_{wr}} & \text{for} \quad 2.38 \times |t\_{cl} - t\_l|^{0.25} < 12.1 \times \sqrt{v\_{wr}} \text{[W/(m}^2 \times \text{K)]} \end{cases} \tag{6}$$

where *var* was set equal to the *va* as occupants were intended to be stationary sensing draught. Equation (5) includes *fcl*, which is calculated by

$$\begin{aligned} f\_{cl} &= 1.00 + 1.290 \times I\_{cl} \text{ for } I\_{cl} \le 0.078 \text{ and} \\ &1.05 + 0.645 \times I\_{cl} \text{ for } I\_{cl} > 0.078 \end{aligned} \tag{7}$$

Continuing the *PMV* index calculation, in Equation (3), evaporative heat exchange at the skin, when the person experiences a sensation of thermal neutrality *Ec* given as

$$E\_c = 3.05 \times 10^{-3} \times \left[ 5733 - 6.99 \times (M - W) - p\_a \right] + 0.42 \times (M - W - 58.15) \text{ (W/m}^2), \tag{8}$$

where *pa* is the water vapor partial pressure [Pa], calculated using measured *RH* by

$$p\_d = \frac{RH}{100} \times 479 + \left(11.52 + 1.62 \times t\_i\right)^2 \text{(Pa)},\tag{9}$$

In addition, Equation (3) for *PMV* includes respiratory convective heat exchange *Cres*, calculated as

$$C\_{res} = 0.0014 \times M \times (34 - t\_i) \text{ (W/m}^2\text{)},\tag{10}$$

and Equation (3) includes also respiratory evaporative heat exchange *Eres*, given as

$$E\_{\rm rcs} = 1.72 \times 10^{-5} \times M \times (5867 - p\_a) \text{ (W/m}^2\text{)},\tag{11}$$

Finally, to predict the rate of people dissatisfied in a thermal environment, the *PPD* index is used. Knowing *PMV*, *PPD* can be calculated as [60]

$$PPD = 100 - 95 \times \exp(-0.03353 \times PMV^4 - 0.2179 \times PMV^2) \tag{12}$$

Measured values are shown in Building result figures in the results chapter and used in Equations (1) and (2) for calculating *Tu* and *DR*, and in Equations (3) and (12) to calculate *PMV* and *PPD*.

#### *2.4. Indoor Climate Questionnaire*

To study occupant satisfaction we provided online questionnaires to the employees of the measured office spaces. As some organizations involved in this study are moving towards policy of a paperless work management, we used Google Forms [61] application. In addition to standard CEN EN 15251 [52] suggestions, we added also questions about age, gender, amount of time behind the desk during workday, and the working environment regarding cabinet or open office plan. The ICQ is presented in Appendix A.

#### *2.5. On-Site Measurements*

This section provides an overview of the TC measurement time and weather information (Table 4), followed by measurement results with calculated TC indicative parameters *Tu*, *DR*, *PMV,* and *PPD*. ICQ survey results are summarized at the end of the results sections.



The experiments were carried out on regular workdays during August. Measurements were taken by two persons, by the main author of this article assisted by graduate students in different buildings. HVAC systems were in regular performance mode without disfunctions or failures recorded. Internal gains by occupants, office equipment, and lighting were in use by default as some desks were empty by unused space, duties, or vacation. No serious defects in HVAC design or construction were observed. Although, some air flow and velocity aspects were noticeable. As in Buildings A and C, active beams were in use, occupants were not always placed sitting according to rule of thumbs, according to the architectural layout, or number of persons. Possible air flow obstacles by lighting fixture (Figure 3a) were noticed with open ceiling in Building A. *DR* risk was also predictable in building E (Figure 3b) where some vanes were taped to closed position. *DR* risk was more carefully considered in Buildings B and D.

**Figure 3.** Possible air flow obstacles with open active beam solution (**a**); modified airflow distribution with fan coil unit (**b**).

#### **3. Results**

Based on on-site measurements, the summary of *va* in each measurement position are shown below for each Building. According to three heights provided in Figure 2, va values during measurement period are shown with box and whiskers plot. Minimum and maximum are at the end of the whiskers, the lower and the upper line of the box are first and third quartiles, the line between is median and the cross shows mean *va* value of the measurement in one position.

On the box and whiskers plot, the category of the indoor climate category is colored according to the lowest criteria achieved during measurements, meaning if one of the three height is in III category, the measurement point is placed in the least, III category. In the table part on the result figures below the box and whiskers plot, measured values and calculated parameters are colored according to the

category reached to be more easily distinguishable. *Tu* and *RH* measured values are not colored as being not categorized. Nonetheless, measurement point indoor climate category is defined by the inferior measured value or calculated parameter reached altogether.

#### *3.1. Building A Results*

The *va* results and TC parameters in Building A equipped with open ceiling active chilled beams are provided below in Figure 4. In Building A, in 2/3 of the measured positions the *va* was below the first indoor climate category threshold. Five positions met the II category requirement and in one position the *va* was above the category II threshold. Measurement No 14 was taken in an office space with unusually high internal gains, where also multi-split fan coil units were additionally added to the environment due to the specifics of the lessee. The results of *ti* and *va* including *DR*, *PMV,* and *PPD* are placed in the first category mainly.

**Figure 4.** Building A air velocity results in measurement points 1–18 and the thermal comfort parameters.

#### *3.2. Building B Results*

The *va* results and TC parameters of Building B with slab-based TABS system are given below in Figure 5. Building B had more measured points in the second category by *PMV* and *PPD* compared to Building A. *DR* met the II category in four measurement positions. Positions 4–8 were in an office, where the ventilation rate had been doubled by the request of the lessee. These four measurements stand out above the others. Regarding the other four buildings observed, displacement ventilation effect can be seen, as *va* fluctuates more near the floor.

**Figure 5.** Building B air velocity results in measurement points 1–16 and the thermal comfort parameters.

#### *3.3. Building C Results*

Building C was equipped with suspended ceiling active chilled beams and the results of *va* and parameters of TC are presented below in Figure 6. *PMV*, *PPD*, and *ti* were similar to Buildings A and B, at the same time *va* and *DR* were measured at two positions in the II category and three times in the III category. The *va* is more fluctuating on the height of the sitting person neck.

**Figure 6.** Building C air velocity results in measurement points 1–19 and the thermal comfort parameters.

#### *3.4. Building D Results*

Equipped with radiant cooling panels, results of *va* and parameters of TC in Building D are showed below in Figure 7. Compared to other buildings, Building D with the least number of positions had the best results on all analyzed parameters. In all cases, I category *DR* was achieved. At all times, mean *va* remained below 0.10 m/s being more fluctuating near the floor.

**Figure 7.** Building D air velocity results in measurement points 1–11 and the thermal comfort parameters.

#### *3.5. Building E Results*

According to the results, Building E achieved the worst TC values by categories. *DR* was in the II category in 4 positions of 14, *ti* was in III category four times. *PMV* and *PPD* second category was not reached 5 times. Fluctuations of *va* were random depending on the height. The *va* results and TC parameters in Building E, with fan coil units mounted in the suspended ceiling, are compared below in Figure 8.

**Figure 8.** Building E air velocity results in measurement points 1–14 and the thermal comfort parameters.

#### *3.6. Results of the Indoor Climate Questionnaire*

Based on ICQ survey, summary of the results for thermal environment are shown below in Figure 9, the ICQ results for *PMV* and *PPD* are presented below in Figure 10. The highest number of answers were in the Building A with 36 responses divided between all age groups equally between men and women. A total of 83% were working in open office layout and 86% were spending most of the day at their workplace. For 83% of the respondents, *ti* was described as suitable. Meanwhile, 6 occupants found it to be warm and 7 slightly cooler. A total of 89% had not or had perceived slight odor, 72% did not find lighting fixtures or sunlight to be disturbing, and 81% found ICQ to be suitable or better. A total of 61% perceived overall acoustics and 36% perceived other noises to be disturbing. Roughly half of the respondents rarely felt eye problems, headaches, or concentration matters and

64% rarely felt nasal or throat irritation. Extra comments mentioned occasional lack of ventilation and air dryness.

**Figure 9.** Indoor climate questionnaire results for indoor air temperature. The descriptions of y-axis are the room air temperature sensation question (upper) and verification questions (middle and lower) from the indoor climate questionnaire (see Appendix A).

**Figure 10.** Indoor climate questionnaire results for predicted percentage of dissatisfied and predicted mean vote.

Respondents in the Building B were 38% females, 2/3 aged between 26–35 or 36–45 and 1/2 spending half of the workday behind the desk. A total of 72% of them working in open office environment. Ninety percent found *ti* to be suitable. A total of 13 of the 29 respondents did not perceive odor. Lighting was disturbing for 21% and sunlight for 14%, meanwhile 14% were dissatisfied with ICQ. Seven percent did not find room acoustics and 17% general noise in office to be disturbing. Half of the respondents had rarely felt eye dryness or irritation, occur headaches or fatigue, and felt nasal problem or dry throat. A total of 62% had rarely felt concentration problems.

Seventy percent of the 20 ICQ respondents in Building C were women. Answers were divided between the age of 26 to 65 with the majority of them working in open office landscape, 2/3 working behind their desk most of the day. Perceived as too warm by 20%, *ti* was suitable by 75% of the occupants. Ninety percent had not perceived or had perceived slight odor. A total of 70% did not find lighting equipment to be disturbing and 75% was not disturbed by the sunlight. Forty percent of the respondents found air quality to be not suitable or unacceptable. A total of 85% perceived colleagues' speech and overall room acoustic to be somewhat disturbing, while 65% claimed other noises to be distracting. A total of 1/3 had rarely felt eye problems, occurred headaches, or tiredness. A total of 45% had rarely felt nasal or throat irritation and 20% had rarely had concentration issues. Extra comments mentioned lower fresh air rate in the end of the day.

Building D had only 8 responses for the online ICQ all of them working in the open office. For the majority of the answers, *ti* was suitable. Odor was rarely noticed, lighting or sunlight was not disturbing. ICQ was suitable or better, while room acoustics was more disturbing than other noises. Nasal issues were more often to occur compared to eye dryness or headaches and concentration issues. Extra comments noted that open office may be cheaper option for the employer being unsuitable for the employees.

A total of 2/3 of the 22 respondents in Building E were in the second age group between 26–35 years and 36% in overall were females. A total of 77% of the tenants were working in an open office environment, while 2/3 of them were spending most of their day behind the desk. One-third found *ti* to be suitable and 2/3 claimed the *ti* to be slightly warm, warm, or hot. Fifty percent perceived weak or moderate odor. Room lighting equipment did not disturb 82% and the sun did not disturb 60% of the respondents. A total of 2/3 marked ICQ suitable, good, or very good. Room acoustic level was not claimed to be disturbing for 40% and other noise for 23% of the respondents. Fifty percent had rarely felt eye dryness or irritation, 64% had rarely occur headaches or fatigue, 82% had rarely felt nasal problems or dry throat, and 50% mentioned concentration issues sometimes, often, or all the time. Extra comments noted that air quality decreases in the second phase of the day and the missing option for opening windows was also described as a disadvantage.

Number of respondents of the ICQ is below the least recommended sample size [63], therefore the results of the ICQ include higher uncertainty (Figure 10). Thermal sensation voted by occupants covers significantly wider range than *PMV* calculated from measurements. Majority of the respondents were working in open office. The most unsatisfying *ti* was in the Building E and the most suitable *ti* was in the Building D. In general, unsuitable *ti* was perceived more as warmer than cooler. In Buildings A, B, and D the *ti* was perceived suitable for over 80% of the employees, while it was 67% in the Building E and the 60% in the Building C.

#### **4. Discussion**

The on-site measurement results showed, that the during cooling summertime *DR* risk can be stated in all observed buildings. Preconception of avoiding fan coil units for cooling does not immediately guarantee a superior thermal environment without draught. However, draught risk was the lowest in Building D with radiant cooling panels as room conditioning units.

Possible causes, *va* and *DR* was not significantly higher in the case of fan coil units in Building E was the taping of air distribution vanes (Figure 3b) and also positioning of the working stations was carried out avoiding direct draught from the fan coil units. This could explain the higher thermal environment temperatures. The induced airflow rate is manually adjustable for open ceiling active chilled beams in Building A and was adjusted into different positions for avoiding possible draught between two beams in various places. In Building C, few suspended ceiling active chilled beams had paper covers blocking air flow from the nozzles. These modifications were made due to the complaints, decrease in productivity or spatial plan and the layout of the workspaces. Described modifications in Buildings A, C, and E refer to possible ineffective floor space areas. Therefore, whether the design or construction may have been inaccurate or user-based thermal environment setpoints do not meet the requirements for *va* and *DR*.

The *va* limit values in EN 16798−1:2019 [54] have been calculated assuming *to* +23 ◦C and *Tu* 40%. Figure 11a illustrates that the *Tu* is considerably higher than the default value, which increases the

unsatisfaction with local TC. However, the measured *to* was higher than the default value in most of the measured positions in all buildings, which decreases the number of dissatisfied. Figure 11b shows that, in general, the *DR* calculated based on measured *to* and *Tu* is in the same scale with the one calculated with the default values.

**Figure 11.** Air velocity and turbulence intensity results according to maximum air velocity categories I–III in summer (**a**); Draught rate correlation in measured and standard-based [54] conditions according to draught rate categories I–III (**b**).

In further analysis of this study *ti* will be more deeply discussed, foreseeing to include transitional period and heating period measurements, façade inspection and *ti* periodical data analysis in the reference buildings. Therefore, the performance of the cooling units according to *ti* could be more clearly presented by period or duration curve. Periodical data analysis on *ti* is mandatory as *ti* presented in this study reflects only a fragment of the thermal environment. Positioning TC measurement values on periodical *ti* data can indicate TC measurement accuracy and dispersion. IQC survey number of respondents also needs additional attention, how to achieve a higher response rate.

There are several limitations to this work. Authors had no control over the boundary conditions during measurements. This study only focuses on a few office spaces in five different building in Tallinn. More further studies of actual work environment need to be performed in order to be able to draw general conclusions about studied room conditioning solutions air distribution performance.

#### **5. Conclusions**

This study was based on TC measurements in open office environments in Tallinn. First or second category measured general thermal comfort in four buildings out of five were still inconvenient for significant number of occupants because of local thermal discomfort caused by draught and by some additional dissatisfaction indicated by questionnaires. Questionnaire survey showed deviation from predicted *PPD* in both directions. Some small occupant groups were either more satisfied or less satisfied at slightly cool or slightly warm thermal sensation, but at neutral sensation the results were more consistent. Less satisfied occupant groups exposed to higher air velocities has likely affected their satisfaction reported in thermal sensation questions because there was no specific draught question available.

Temperature measurements showed that air and operative temperature was the worst in Building E which was close to drop out from category III, while measurement results in Buildings A–D remained in between I and II category. According to the questionnaire over 80% of the employees in Buildings A, B, and C, and 75% in Building D were thermally satisfied. In the Building E, 59% of occupants found the thermal environment unsuitable or unacceptable. Generally, the average thermal satisfaction of occupants was well in line with the measurements.

Air velocity and draught rate measurements showed that modern offices do not necessarily reach to generally expected good indoor climate category II air velocity and draught rate values. A room conditioning solution with suspended ceiling active chilled beams in Building C, displacement ventilation in Building B with TABS and fan coil units in Building E showed category III performance only. Open ceiling active chilled beams in Building A corresponded to category II requirements and ceiling panels for room conditioning in Building D showed superior Category I performance. Category II and III results with active chilled beams indicate that dedicated air distribution solution together with proper design and sizing is needed to reach category II.

We found that existing standards do not provide enough detailed questionnaire for the assessment of occupant dissatisfaction. Our results suggest that questionnaire could be an easier compliance assessment method compared to measurements, which need an expensive equipment and carefully selected measurement days. For the compliance assessment with the measurement, there is more guidance needed especially how to select relevant measurement conditions and locations for draught rate measurement. Future office buildings with open-plan layouts revealed to be demanding environments where careful air distribution design is needed in order to meet comfort requirements.

**Author Contributions:** J.K. conceived and designed the experiments. M.K. prepared agreements with the building owners, performed the measurements and analyzed the data. M.T. and J.K. helped to perform the data analysis. M.K., R.S., M.T., and J.K. wrote this paper. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the Estonian Centre of Excellence in Zero Energy and Resource Efficient Smart Buildings and Districts, ZEBE (grant 2014-2020.4.01.15-0016) funded by the European Regional Development Fund, by the programme Mobilitas Pluss (Grant No—2014-2020.4.01.16-0024, MOBTP88), by the European Commission through the H2020 project Finest Twins (grant No. 856602) and the Estonian Research Council grant (PSG409).

**Acknowledgments:** The authors are grateful for the provided cooperation of the building owners, questionnaire respondents for their time and the valuable help from Tallinn University of Technology graduate students.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Nomenclature**



#### **Appendix A**

The ICQ form is for online survey is provided below.


#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## *Review* **Determination of Thermal Comfort in Indoor Sport Facilities Located in Moderate Environments: An Overview**

#### **Fabio Fantozzi and Giulia Lamberti \***

DESTEC, Dept. of Energy, Systems, Territory and Constructions Engineering, School of Engineering, University of Pisa, Largo Lucio Lazzarino, 56122 Pisa, Italy; f.fantozzi@ing.unipi.it

**\*** Correspondence: giulia.lamberti@phd.unipi.it

Received: 31 October 2019; Accepted: 29 November 2019; Published: 3 December 2019

**Abstract:** In previous years, providing comfort in indoor environments has become a major question for researchers. Thus, indoor environmental quality (IEQ)—concerning the aspects of air quality, thermal comfort, visual and acoustical quality—assumed a crucial role. Considering sport facilities, the evaluation of the thermal environment is one of the main issues that should be faced, as it may interfere with athletes' performance and health. Thus, the necessity of a review comprehending the existing knowledge regarding the evaluation of the thermal environment and its application to sport facilities becomes increasingly relevant. This paper has the purpose to consolidate the aspects related to thermal comfort and their application to sport practice, through a deep study concerning the engineering, physiological, and psychological approaches to thermal comfort, a review of the main standards on the topic and an analysis of the methodologies and the models used by researchers to determine the thermal sensation of sport facilities' occupants. Therefore, this review provides the basis for future research on the determination of thermal comfort in indoor sport facilities located in moderate environments.

**Keywords:** thermal comfort models; thermal comfort assessment; Fanger's models; moderate environments; sport facilities

#### **1. Introduction**

In recent years, ensuring comfort in indoor environments has become a real challenge involving different disciplines. However, in the past, the parameters used to guarantee comfort and the approaches to improve the quality of the indoor environment were often studied separately [1]. Furthermore, comfort in indoor spaces can be ensured through the control of all the environmental factors, which include indoor air quality, thermal comfort, lighting, and acoustical quality [2]. In this context, the indoor environmental quality (IEQ) that includes all these aspects, assumes a fundamental role in the determination of the conditions of comfort in buildings. Since the time that humans spend indoors has largely increased, several studies have been carried out in order to enhance comfort in indoor spaces, especially in offices, schools, hospitals, etc. The focus of researchers was often based on possible improvements of air quality [3] or on the reduction of the impact of pollutants in indoor air [4]. Visual quality and acoustical comfort have been also studied in workplaces and in educational rooms [5–8], since they may have a great impact on the focus and on the performance of the occupants. Finally, thermal comfort has been often studied in relation to the characteristic of the envelope and of the internal structures [9], as the thermal behavior of the building can largely influence the environmental conditions indoors, which also has an impact on the comfort and performance of people.

In sport facilities, these four aspects have been studied in order to improve the comfort and the performance of the athletes. In particular, light has been recognized as one of the most important factors to ensure the correct practice of the sport [10,11], while indoor air quality has been considered fundamental for ensuring health of the athletes [12]. Then, since swimming pools and sport halls are environments in which noise level and speech intelligibility can determine the comfort or the safety of the occupants, research on the assessment of acoustic conditions and on the use of acoustic treatments to improve the quality of the environment have been carried out [13].

The thermal environment is probably the most important parameter that should be considered when performing sports, as it can determine the safety and the performance of the athletes. In moderate environments, defined as spaces in which it is possible to reach the condition of thermal well-being, only few studies have been carried out. These studies were focused on the monitoring the thermal conditions according to thermal comfort indices [14,15], on the interventions to improve thermal comfort [16,17], on the comparison between objective and subjective measurements [18,19], on the assessment of thermal comfort to balance energy use [16,20] and on the association between thermal comfort and physiological responses during exercise [21,22]. However, there was no standardization in the measurement methodologies or in the models that were used to predict thermal sensation in sport facilities. Even the norms regarding the perception of the thermal environment do not often consider the parameters that should be maintained in sport halls and swimming pools and only in some cases Sports Federations provide these values, even if they are often incomplete.

The main purpose of this article is to consolidate the existing knowledge regarding the thermal environment and its application to sport facilities. In order to achieve this result, it was necessary to use a multidisciplinary approach that considers all the aspects of thermal comfort, from the engineering approach, which treats man as a heat engine, to the physiological and psychological ones, which also play a key role in the perception of the thermal environment, especially during sport practice. Then, in order to consider the practical aspects of the prediction of the thermal sensation of the athletes, the main standards and Federations' norms have been reviewed, as well as the models used by researchers to evaluate thermal comfort in sport facilities. Finally, the methodologies developed to assess thermal comfort in these environments have been studied. This paper lays the groundwork for future research on the determination of indoor thermal comfort in sport facilities located in moderate environments, as in these spaces thermal conditions have a fundamental role in the performance and in the health of the athletes.

#### **2. Thermal Comfort Approaches**

The perception of the thermal environment can be considered dependent on several factors, derived from different fields of research. In particular, three main approaches have been identified: engineering, physiological, and psychological approaches. The engineering approach is based on the representation of the humans as 'heat engines', who can exchange heat with the environment and it implies that the thermal sensation is dependent on the heat balance of the human body. The physiological approach considers instead the mechanisms with which the body responds to the thermal environment (e.g., thermoregulatory responses). Finally, the psychological approach concerns the psychological phenomena regarding the individuals' perception of a certain environment [23].

#### *2.1. Engineering Approach*

In the engineering approach, the human body is represented as a heat engine, which can give or receive heat from the environment through conduction, convection, radiation, and evaporation. The heat in the body is produced by the metabolic processes occurring during human life. The heat exchange between the body and the environment can be determined through the heat balance equation [23]

$$\mathbf{M} - \mathbf{W} = \mathbf{C}\_{\mathbf{k}} + \mathbf{C} + \mathbf{R} + \mathbf{E} + \mathbf{S} \tag{1}$$

where M is the metabolic rate of the body (W), W is the mechanical work (W), Ck is the heat transfer by conduction (W), C is the heat transfer by convection (W), R is the heat transfer by radiation (W), E is the heat transfer by evaporation (W), and S is the heat storage (W).

In conditions of thermal equilibrium, the heat storage is null (S = 0) and the heat balance equation can be written as

$$\mathbf{M} - \mathbf{W} - \mathbf{C}\_{\mathbf{k}} - \mathbf{C} - \mathbf{R} - \mathbf{E} = \mathbf{0} \tag{2}$$

Note that this equation is generally applied to steady state conditions and it should be carefully adopted during sport practice, as exercise is usually performed under transient conditions. Figure 1 reports the mechanisms of heat transfer during exercise.

**Figure 1.** Heat transfer mechanisms during sport activity. The body can exchange heat with the environment through conduction, convection, radiation, and evaporation. The production of external work through muscular activity leads to an increase of the heat that has to be dispersed in the environment.

#### 2.1.1. DuBois Area

The heat produced by the body flows through the body surface. A method for the calculation of the nude body surface area is given by DuBois formula [24]

ADB = 0.2025 W0.425 H0.725, (3)

where W is the weight (kg) and H is the height (m) of the body. Generally, the value of ADB = 1.8 m<sup>2</sup> is assumed.

#### 2.1.2. Heat Exchange through Conduction

Generally, the heat exchange between the body and the environment through conduction is limited, as it involves small parts of the body. Therefore, the conductive effects are often neglected, or included in the convective effects [25]. However, conduction must be considered in the heat balance when the body is in contact with large surfaces. In this case, the heat loss or gain is dependent on factors such as the body and surface temperatures, the area of contact and the conductivity of the surface and of the body tissues [26]. During sport activity, heat exchange through conduction can occur for example in running, when the athlete is running on a hot road, or in cycling, when the athlete is in contact with the seat of the ridden bicycle.

#### 2.1.3. Heat Exchange through Convection

Heat transfer through convection totals up to 15% of the whole heat loss in stationary conditions, but even more when the air is moving over the body surface [26]. During sport activity, convection can occur due to the body movement, which generates air (e.g., in running, riding, etc.) or water (e.g., swimming) currents or due to the air movement (e.g., wind). The air movement around the skin is responsible for convective cooling.

Heat transfer by convection is given by [25]

$$\mathbf{C} = \mathbf{h}\_{\mathbb{C}} \left( \mathbf{T}\_{\text{sk}} - \mathbf{T}\_{\text{a}} \right) \mathbf{A}\_{\mathbb{C}} \ \mathbf{f}\_{\text{cl}} \tag{4}$$

where hc is the convective heat transfer coefficient (W/m2 K), Ta is the air temperature (K), Tsk is the mean skin temperature (K), Ac is the body surface involved in the heat exchange through convection (m2) (Ac <sup>≈</sup> ADB) and fcl is the clothing area factor.

The clothing area factor (fcl) can be calculated as [27]

$$\mathbf{f\_{cl}} = \mathbf{1.00} + \mathbf{0.28} \,\mathbf{I\_{cl}} \tag{5}$$

where Icl (clo) is the thermal insulation of clothing, whose values are provided for everyday garments in the tables reported in the ISO 9920. Movement tends to let the insulating characteristics of the clothing and of the boundary air layer decrease. In warm environments, where convective heat loss has a positive effect, fabrics are developed in order to let the air to flow between the body and the garments. Conversely, in cold environments, clothing is designed in order to minimize the air movement, preventing convective heat transfer and maintaining body warmth [28].

The convective heat transfer coefficient (hc) is a function of several parameters such as the velocity of the currents, density, and viscosity of the fluid involved and the shape of the exposed surface. An approximate value of hc is given by [25]

$$\begin{array}{ll} \mathbf{h\_{c}} = 3.5 + 5.2 \text{ V}\_{\text{ar}}, & \text{ for } \text{V}\_{\text{a}} \le 1 \text{ m/s} \\ \mathbf{h\_{c}} = 8.7 \text{ V}\_{\text{ar}} \,^{0.6}, & \text{ for } \text{V}\_{\text{a}} > 1 \text{ m/s} \end{array} \tag{6}$$

where Va is the air velocity (m/s), Var is the resultant air velocity (m/s) considering the environmental air velocity and the movement of the person and it can be calculated as [25]

$$\mathbf{V\_{ar}} = \mathbf{V\_{a}} + 0.0052 \text{ (M} - 58),\tag{7}$$

where M is the metabolic heat production (W/m2), with the condition that it is considered M = 200 W/m<sup>2</sup> when M exceeds the value of 200 W/m2.

The influence of the human body's movement on heat exchange can be considered through the calculation of the convective heat transfer coefficient. Several studies have been carried out on this topic, analyzing standing and seating postures and different air speeds occurring due to the movements of mannequin simulating walking and running [29,30] or to the wind [31]. Further research has been developed using computational fluid dynamics to assess the convective heat transfer of individual body segments for cyclist positions [32]. Moreover—since water convection is the only important heat transfer mechanism—in the past, several studies have been performed in order to determine hc analytically [33], or on a heated copper manikin located in the water [34], or detected on experimental data on humans [35].

#### 2.1.4. Heat Exchange through Radiation

Thermal radiation is considered to be one of the factors that can influence the most the heat exchange during sport activity [26]. Only in water sports, the component of radiative heat loss is usually negligible [36]. Since body temperature during exercise is generally higher than the air temperature, there is a loss of radiative heat energy from the body. Only in warm environments, where the air temperature may be higher than the skin temperature, the body can gain heat through radiation.

The heat loss through radiation is given by [25]

$$\mathbf{R} = \mathbf{h}\_{\rm r} \left(\mathbf{T}\_{\rm sk} - \mathbf{T}\_{\rm r}\right) \mathbf{A}\_{\rm r} \mathbf{f}\_{\rm cl} \tag{8}$$

where hr is the radiative heat transfer coefficient (W/m2K), Tr is the mean radiant temperature (K), Tsk is the mean skin temperature (K), Ar is the effective radiation area of the body (m2) and fcl is the clothing area factor.

hr can be calculated as

$$\mathbf{h\_{r}} = 4\sigma\varepsilon\_{\rm sk} \left(\frac{\mathbf{T\_{r}} + \mathbf{T\_{sk}}}{2}\right)^{3},\tag{9}$$

where <sup>σ</sup> <sup>=</sup> 5.67 <sup>×</sup> <sup>10</sup>−<sup>8</sup> <sup>W</sup>/m2K4 is the Stefan-Boltzmann coefficient, <sup>ε</sup> is the emissivity of the body (for the skin ε = 0.97–0.98).

Ar, the effective radiation area of the body is given by [25]

$$\mathbf{A\_{r}} = \left(\mathbf{A\_{r}/A\_{\rm DB}}\right)\mathbf{A\_{\rm DB}}.\tag{10}$$

where Ar/ADB = 0.67 (for squatting position)—0.70 (for sitting position)—0.77 (for standing position).

#### 2.1.5. Heat Exchange through Evaporation

Heat loss through evaporation can occur through skin (by passive diffusion or sweating) and respiratory system (by breathing). Under steady state conditions, it accounts 10% to 25% of the total heat loss and it depends on factors such as relative humidity of the environment, air and skin temperature, air velocity, and clothing [37]. During sport activity, thermoregulation depends mainly on the heat loss through evaporation of sweat and it can arrive to account up to 90% of the total heat loss [38]. In water sports, evaporation cannot be considered as a mechanism of heat exchange [36].

Heat exchange through evaporation can be calculated as [25]

$$\mathbf{E} = \mathbf{h}\_{\mathrm{e}} \left( \mathbf{P}\_{\mathrm{skH2O}} - \mathbf{P}\_{\mathrm{a}\mathrm{H2O}} \right) \mathbf{A}\_{\mathrm{e}} \, \mathrm{F}\_{\mathrm{pcl}\prime} \tag{11}$$

where he is the evaporative heat transfer coefficient (W/m<sup>2</sup> Pa), PaH2O is the water vapor pressure in the environment (Pa), PskH2O is the water vapor pressure in saturated air at Tsk (Pa), Ae is the evaporative surface (m2), and Fpcl is the clothing permeability factor.

he can be calculated as [25]

$$\mathbf{h\_{e}} = \mathbf{k} \,\, \mathbf{h\_{c}} \tag{12}$$

with k = 16.7 K/Pa

Ae can be calculated as

$$\mathbf{A\_e = (A\_e/A\_{DB})\ A\_{DB} = w\ A\_{DB\prime}} \tag{13}$$

where w is the skin wittedness, which is a physiological index defined as the ratio between the actual sweating rate and the maximum sweating rate that occurs when the skin is completely wet. w can range from 0.06, when the evaporative heat loss is caused only by passive diffusion, to 1, when the skin surface is completely wet.

#### 2.1.6. Strategies Adopted from Athletes using Heat Transfer Mechanisms to Support Thermoregulation

The heat transfer mechanisms may support thermoregulation and improve sport performance [39]. Conduction is often used when an athlete is warm to decrease his body temperature. In particular, possible solutions are to put him in contact with cold surfaces or to let him wear special clothing such as ice vests. When the athlete is cold, wearing sport garments that present good thermal insulation may

prevent the heat flow from the skin to the environment. In fact, conduction is particularly important when designing sport equipment, especially when it is composed by conductive materials, as for example the baseball bats or the motor racing seats. In this case, it is important to maintain the equipment at a temperature that is safe for the athletes.

Convection is an effective method to decrease body temperature and it can be supported by the use of fans that increase the heat flow and cool him down. On the contrary, the use of wind-breaker jackets may prevent excessive body cooling when the athlete is cold.

Heat loss through radiation can be increased by increasing the skin exposure to the environment or decreased by exposing the athlete to the sun or to other radiation sources.

Finally, evaporation is a fundamental heat transfer mechanism during exercise, as the body can produce a great amount of sweat. If an athlete is warm, pouring the water over the body can be an efficient way to decrease his temperature, as it leaves more water on the skin to evaporate. Conversely, if an athlete is cold, solutions to prevent the sweat evaporation include removing the water from the skin, removing wet clothes, or wearing additional clothes.

#### 2.1.7. Use of Sport Garments to Control Heat Transfer Mechanisms

The presence of clothing on the human body has several implications on the heat balance. In particular, when considering sport garments, the selection of certain materials and design play a key role in the performance of the athletes. In particular, sport clothing must provide thermo-physiological comfort, supporting the wearer's thermoregulation, keeping the wearer at a comfortable temperature and maintaining the micro-climate between skin and textile as dry as possible [40]. For this reason, understanding the requirements of each sport is an essential step in the design of sport apparel and different studies have been performed for specific sports as for example baseball [41], snowboarding [42], rowing [43], athletics [44], or fitness [45]. The importance of sport clothing is evident when the protection from the environment is required for survival (e.g., mountain sports), but even in common applications it can have a fundamental role in athletes' performance. The aim of sports garments is in fact to provide a comfortable microclimate for the athletes, since comfort may affect their sport performance as it can avoid them from using more reserves in order to maintain the heat balance with the environment [46]. The aim of the study on sport garments is particularly relevant as, in some sports, a uniform is required (e.g., fencing), which does not allow the athletes to modify their conditions, and to adapt to the thermal environment.

The aspects that must be considered with regard to the thermal performance of clothing are the thermal and the moisture management. The early versions of performance garments consisted of a three-layer system, constructed with a base layer with the function of managing the moisture, a middle layer necessary for the insulation and a protective outer layer [47]. Even if this system was primarily used for outdoor apparel; nowadays, it is often adopted in the production of garments for indoor sports, with specific adaptations in order to improve performance. In fact, the necessities of the athletes may be different according to the environment in which they are exercising, as in moderate climates the heat production is high and heat generally flows from the body to the ambient, while in severe environments (both cold and hot) the mechanisms of heat exchange may be different. Moreover, professional athletes may have additional requirements for their apparel, as they have to exchange a great amount of heat with the environment, due to their high metabolic rates. For this reason, the composition of the garments is particularly relevant, as it can determine the heat exchange preventing the heat transfer by conduction, maintaining still air in the clothing, and managing moisture.

The mechanisms of heat transfer through clothing are shown in Figure 2. Regarding the heat exchange through conduction, the presence of garments on the body reduces the amount of heat loss and the characteristics of the clothing may affect the way in which the heat is exchanged [48]. Substantially, the heat transfer through conduction occurs between the inside and the outside of the clothing. In hot environments, apparel should be composed by high conductive materials, in order to let the heat flow from the body to the surroundings, while in cold conditions they should present low

conductivity and they are required to have layer that can trap air, in order to improve their thermal resistance [28]. During sport activity, convection may occur between the apparel (or the skin) and the environment. In warm conditions, the main necessity is to cool down the body, therefore apparel is designed to allow the air flow between the body and the garments, while in cold environments the movement of air should be minimal [28]. Considering the sport of swimming, the optimization of the convective flux has not only the function of improving the heat exchange, but it can enhance the performance thanks to the better hydrodynamics [49]. With regard to the heat exchange through radiation, thermal insulation of the clothing can lead to a decrease of this heat flux, through the reduction of the temperature difference between the skin and the surrounding area. In particular, color and texture of sport clothing are specifically relevant to the heat gain and loss through radiation [28]. For this reason, several studies have been performed in order to produce clothing with metallic coatings or finishing technology that can shield infrared radiation [49]. Finally, since evaporative heat loss is specifically relevant during sport activity, it is important to take it into consideration when designing sport garments. In particular, in warm climates, clothing should be conceived to transport the sweat on their outer layer in order to let it evaporate, thus reducing the body temperature. In cold environments the athletes must also sweat, even if the evaporation of moisture is not intended, thus fibers that draw sweat to the outside of the apparel or to the internal microclimate of clothing are generally used [28].

**Figure 2.** Heat transfer through clothing (modified from [50]). Heat flows from the internal to the external part of the clothing through conduction and then it is exchanged with the environment through conduction, convection, radiation, and evaporation.

#### *2.2. Physiological Approach*

The physiological approach concerns the mechanisms with which the body reacts to the thermal environment (e.g., thermoregulatory responses).

#### 2.2.1. Heat Production during Exercise

The estimation of the heat produced by the metabolism is fundamental to assess the human thermal environment. The free energy necessary for living processes comes from the food and is then converted in the body cells thanks to the ATP-ADP cycle (adenosine triphosphate–adenosine diphosphate cycle) for ensuring life processes and for producing internal and external work. Internal mechanical work consists of the processes that take part in the body, such as the blood circulation, the movement of the air through the lungs or the work of the heart, while external mechanical work is a consequence of muscular contraction [51]. The metabolic heat is a waste product of the metabolism and it must be dispersed in the environment, in order to maintain the body temperature constant (Figure 3).

**Figure 3.** Transformation of free energy into work and heat (modified from [51]). The free energy coming from food is transformed into internal and external work and it is fundamental to guarantee body integrity. However, due to the inefficiency of the systems, part of the energy is converted into heat, which must be released in the environment.

The energy expenditure is distinguished in basal metabolism (at rest) and energy metabolism (when muscular work occurs). The basal metabolism is used for vital processes such as cerebral, circulatory, or respiratory activities and it is dependent on several factors—including sex, age, or hormonal activity—while the energy metabolism occurs during muscular work and it depends on the intensity of the work, the speed and the duration of the muscular contraction. The measurement of the metabolic activity is performed through the detection of the oxygen consumption. In particular, 1 l of O2 corresponds to 21.1 kJ if carbohydrates are oxidized or to 19.6 kJ if lipids are oxidized [51].

During sport activity, muscular contraction takes place thanks to the energy released by ATP. However, the ATP reserves are sufficient only for work lasting about one second. For this reason, the body presents some energetic mechanisms able to re-synthetize ATP. There are three main energy systems for ensuring these mechanisms [52]:


Of these, it is important to consider the power (maximum amount of energy produced), capacity (total amount of energy produced), latency (time necessary to obtain the maximum power), and resting

time (time necessary for the reconstitution of the system). Table 1 shows the characteristics of each of these energy systems.


**Table 1.** Characteristics of the three energy systems to re-synthetize ATP.

In general, these energy systems during sport activity do not occur separately, but they intervene together. As the number of sport activities is vast, it can be fundamental to describe the kind of mechanism occurring through the time necessary to perform certain movements (Table 2). For example, a basketball match has a duration of 40–48 min; therefore, considering what previously stated, it would mean that the metabolic system involved is the aerobic. However, in basketball rapid and intense movements occur, which involve also the anaerobic systems. Therefore, basketball and other sports such as fencing, baseball, football, golf, hockey, tennis, or volleyball involve both aerobic and anaerobic phases. In other sports such as swimming, running, skiing, or rowing, the energetic system used depends on the duration of the competition. For example, as swimming 200 m and running 800 m require the same time, the energetic system used by the body will be the same.

**Table 2.** Duration of the performance in relation to the energy system (modified from [52]).


The measurement unit of the metabolic rate is the Met; 1 Met corresponds to the metabolic rate at rest.

#### 1 Met = 58 W/m2

Standards report the metabolic rate for different activities and the way in which they can be calculated [53] while the 2011 Compendium of Physical Activities [54] shows the metabolic rate for different sports. Table 3 displays the metabolic rate corresponding to different activities.


**Table 3.** Typical metabolic rates for different activities (modified from [54]).

#### 2.2.2. Thermoregulation during Exercise

In humans, temperature is regulated through control systems that ensure homeostasis through behavioral and physiological mechanisms of thermoregulation. The first includes all the tools that humans can use to support their thermal comfort, such as the choice of an appropriate clothing or the adjustment of the indoor environmental conditions (opening/closing a window, use HVAC systems, etc.). The second consists of several physiological mechanisms which can intervene to maintain homeostasis, which are the vasomotor response (vasoconstriction or vasodilation), sweating, and shivering. The physiological thermoregulation is a feedback system: temperature receptors are located in the skin and they are connected to the hypothalamus, which has the function of providing homothermia and can activate the mechanisms of thermoregulation through nervous pathways (Figure 4). The physiological field of thermoregulation is generally wider than the zone of thermal neutrality which also represents the zone of thermal comfort. Therefore, behavioral thermoregulation occurs earlier than physiological thermoregulation.

**Figure 4.** Human thermoregulatory system (modified from [55]). Temperature receptors located in the skin are connected with the hypothalamus, which compares the temperature of the body with a reference temperature and it can activate thermoregulatory systems in order to maintain body temperature constant.

During sport practice, the heat production of the body may exceed 1000 W, thus the body temperature tends to increase. In fact, only a modest part of the heat produced by muscles is initially transferred to the environment and most of it increases body's internal temperature. For example, it has been demonstrated that during intense cycle exercise, the temperature can rise up to 1 ◦C/min [56]. This heat storage cannot be maintained for long periods, or the athletic performance would be compromised due to the overheating and heat exhaustion. When the temperature reaches a certain limit, the thermoregulatory systems occur and the heat is dissipated though vasodilation and sweat. These mechanisms do not occur only in warm environments, but in every condition when the physical exercise is intense enough. In fact, muscular usage generally increases body temperature, usually resulting in temperatures higher than the environmental temperature.

Training can improve thermoregulation during sport practice, leading to an increase of sweat rate and skin blood flow. In fact, elite performers usually present an augmented sweat secretion that occurs in the early phase of exercise, leading indeed to a fast dehydration. Furthermore, professional athletes show an increase of blood total volume and maximal cardiac output, resulting in a better heat dissipation through vasodilation that leads the temperature decrease thanks to the convective cooling [57]. Acclimatization plays also a crucial role in sport performance, especially when competitions take place in different environments from where athletes are used to train.

Gender and age differences can also play a key role in thermoregulation, since physiological properties (e.g., sex hormones, exercise capacity, etc.), anthropometric characteristics (e.g., body mass and size), body composition (e.g., muscles and body fat), and physical activity level may be different. In general, women and elderly people have a lower sweat capacity and a higher core temperature than men [58] and therefore they present less endurance because of heat exposure. Actually, it has been observed that the human response to exercise depends mostly on the aerobic capacity (VO2max), which is also correlated to factors such as gender and age [59].

Finally, another aspect that may affect thermoregulation is clothing, as it represents an additional layer that may delay the heat transmission through conduction or prevent the sweat evaporation. However, research has shown that in warm and in moderate environments, garments do not have any effect on the thermoregulatory response. In fact, the addition of layers or the fabric characteristics seem not to affect the physiological responses of the body [60]. On the contrary, in cold conditions clothing may influence thermoregulation and in this case the ideal clothing can block air movement but allows the passage of water vapor when the production of sweat occurs [61].

#### 2.2.3. Body Temperature during Exercise

The human body can be considered divided in two parts, a core (inner part) and a shell (outer part). The temperature of the central core in stationary conditions is maintained stable by the thermoregulatory systems activated by the hypothalamus that maintains homeostasis, as the internal temperature cannot exceed certain limits, or the vital organs would result compromised. The shell temperature is usually defined by the mean temperature on the skin and it can vary according to the environmental conditions. The variation of the body temperature depends on several factors such as the environmental conditions, the thermoregulatory system and the metabolic rate, which determine the heat production of the body. During sport activity, the core temperature can rise up to 40 ◦C, due to muscular strain which determines a large amount of metabolic heat production. Temperatures higher than 40 ◦C may cause performance break-down or, in extreme cases, health problems [62]. Skin temperature has instead a different trend that it is usually inversely proportional to the exercise intensity, at least at the onset of exercise [63]. Recent studies on runners showed that in the first phase of exercise the body presents a decrease of skin temperature due to the vasoconstriction while, as soon as the core temperature reaches the threshold values, the warmer blood is directed to the shell, leading to an increase of skin temperature and to a decrease of core temperature [64]. However, since different sports involve the use of specific muscles, in the non-active regions the skin vasoconstriction is particularly evident, while in

the active ones the skin temperature increases earlier due to the thermal conduction from the active muscles to the skin surface above them [65].

The evaluation of body temperature during exercise is fundamental to ensure a good performance and healthy conditions to the athletes. In extreme cases, when the body temperature gets too high (hyperthermia) or too low (hypothermia), accidents may occur, as it can happen in warm and humid environments or in cold spaces. Warm and humid environments are particularly critical for athletes [52], since their thermoregulatory system cannot properly operate (high temperature prevents heat transfer by convection and radiation and high humidity levels do not allow the sweat evaporation). For this reason, the assessment and the control of core and skin temperatures have a primary importance in the performance and in the safety of the athletes.

#### *2.3. Psychological Approach*

The thermal sensation perceived by humans derives from the sensory experience, therefore it cannot be based only on physical or physiological approaches. In fact, often the environmental factors are not always the cause of thermal dissatisfaction in buildings [66]. Even people's expectations may influence their satisfaction, as occupants can be satisfied with a certain environment because they do not expect any better condition, or be dissatisfied because they would expect a different environment. For example, elite athletes may have different expectation than other athletes, as they are more used to high quality environmental conditions. Thus, a certain environment could be considered satisfactory or not according to the personal experience of the single athlete and not for the physiological or environmental conditions.

#### 2.3.1. Thermal Sensation

Thermal sensation is related to physical and physiological aspects, but also to psychological features, as it is related to how humans feel in a certain environment. It is important to distinguish how a person feels and how he or she would like to be (warmer/colder) or how a certain environment can be described. In particular, physical exercise can affect the thermal response of the athletes, as they may feel warm in environments in which they would perceive cold feelings in conditions of rest. The research of McIntyre [67] shows that usually cold sensations are determined by mean skin temperature, while warm feelings occur initially due to skin temperature and then due to core temperature and they are closely related to the skin wettedness. Furthermore, it has been demonstrated [68] that skin temperature, which may affect the thermal sensation of the athletes, is closely related to environmental conditions for a large range of exercise (from 30% to 70% VO2max). Acclimatization can also play a fundamental role in the perception of the thermal environment, reducing the warmth sensation up to 70–80% during sport practice where air temperature was maintained 50 ◦C [68].

#### 2.3.2. Thermal Discomfort

Thermal discomfort during exercise has an important role in sport performance, since it may affect the sense of effort of the athletes. Warm discomfort appears when physiological mechanisms such as vasodilation and sweat secretion occur, but it is also dependent on factors like body temperature and skin wittedness [69]. In particular, it was shown that during exercise in thermal equilibrium the level of skin wittedness providing a sensation of comfort rises from 0–10% to 20–25%, showing the influence of physical activity on human feelings. Moreover, when considering steady-state exercise, warm sensations are generally reduced when the athletes are well trained; otherwise, thermal discomfort was usually associated with the sweat rate and to the vasomotor response [68]. On the contrary, cold discomfort occurs due to vasoconstriction and to the consequent reduction of skin temperature [69]. However, this situation is generally related to sport practice in cold environments, since the majority of heat produced by the body during exercise leads usually to warm sensations.

#### **3. Standards**

Thermal environments are divided into moderate, hot, and cold. For moderate environments, the reference standard is the UNI EN ISO 7730 [70], which reports the indices that can be used to evaluate the perception of the thermal environment in indoor spaces. Furthermore, general standards explain the main concepts regarding the thermal environment, including the calculation of clothing insulation, the assessment of metabolic rate for different activities, the methods to conduct objective, and subjective measurements and the explanation of the physiological responses of humans. Figure 5 shows an overview on the current normative regarding thermal comfort, even if there is to consider that in this field, the normative is continuously evolving, as technical and scientific knowledge is rapidly developing.

**Figure 5.** Standards for thermal environments: an overview (modified from: [55]). General standards can be applied to all environmental conditions, while the others can be used for the calculation of specific indices developed for different thermal environments.

With regard to sport facilities, generally they can be very heterogeneous, therefore specific literature and standards for the regulation of the parameters that should be maintained in these environments are needed. In fact, often different activities are carried out in these spaces as they are usually multifunctional buildings in which diverse sports are performed, hence the difficulty of finding a unique norm that handles all these aspects. Since the legislation regarding construction and maintenance of sport facilities is often related to hygienic conditions, the normative varies among countries. For example, in the USA the first standard concerning the thermal aspects in indoor environments is the ASHRAE 55/2004, regarding "Thermal Environmental Conditions for Human Occupancy" [71]. However, this standard does not provide any specific value to be maintained in sport facilities and usually different States present diverse regulations on these aspects. In Russia, the SNIP 31-112-2004 "Physical Training and Sports Halls" [72] reports the values of air temperature, relative humidity, and air velocity that should be maintained in an "ordinary sports hall", where different activities can be carried out. Therefore, this standard does not provide any specific information regarding the values to be maintained in different sport facilities, in relation to the sport performed. In Europe, a unique standard regarding thermal comfort in sport facilities does not exist and the

regulations may vary among the countries, as it happens in the USA. In particular, in Italy, the most important standard managing thermal comfort in sports halls is the guideline of CONI [73], which defines indications about air quality, thermal, lighting, and acoustic environment in sport halls and swimming pools. Table 4 shows the main values to be maintained in indoor sports halls and swimming pool, according to CONI's guidelines.


**Table 4.** Environmental parameters for sports halls and natatorium facilities (modified from [73])

Notes: (1) In the table are reported only the values concerning the thermal environment. The complete table can be found in [73]. (2) The values refer to the case of artificial ventilation. (3) At least 20 m3/hour/person at maximum crowding for the spectator's area; 30 m3/hour/person for the space occupied by the athletes. (4) Values to be established in relation to the thermo-hygrometric characteristics to be achieved. (5) For the water temperature in the pools, specific values are given by CONI's guidelines. (6) The temperature of the air in the changing rooms (excluding those of the swimming facilities) is appropriate to be 2–4 ◦C higher than that of the sport room. (7) The temperature of the water in the showers, must not be lower than 37 ◦C and not higher than 40–48 ◦C. (8) The thermo-hygrometric, ventilation and lighting engineering requirements must conform to what is indicated in the Agreement of 16 January 2003—between the Minister of Health, the Regions and the autonomous provinces of Trento and Bolzano on the sanitary aspects for the construction, maintenance, and supervision of the swimming pools.

However, since different sports present diverse requirements, International Federations often provide not only the rules regarding the game and the materials, but also standards concerning environmental parameters such as air temperature, relative humidity, and air velocity that should be maintained indoors for each sport. In Table 5 the environmental parameters provided for indoor sport facilities by federations recognized by the International Olympic Committee (IOC) are reported. It can be noticed that most federations show at least the values of air temperature that should be maintained in the playing area. However, several sport do not present any environmental value (e.g., boxing, fencing, etc.), therefore specific studies should be performed in order to define these parameters and to establish more precise values for the existing ones, since several sport federations report only the value of air temperature and they do not consider, for example, relative humidity, and air velocity.


**Table 5.** Environmental parameters to be maintained in the playing area given by sport federations.

#### **4. Thermal Comfort Models Applied to Sport Facilities**

Sport facilities present high complexity, due to the different activities that are carried out in these environments. For this reason, experimental methods have been developed by researchers in order to predict or to assess the thermal conditions in swimming pools and sport halls. However, only a few studies has been developed, as shown by literature review displayed in Table 6.

**Table 6.** Literature review of the scientific papers regarding thermal comfort in sport facilities.



**Table 6.** *Cont*.

#### *4.1. Human Thermal Physiological Models*

In order to determine the thermal comfort, several physiological models have been developed, from the one-node model, representing the complete human body as one node [90] to the more complex and realistic ones, representing the body with a multi-elements model using finite elements [91]. In sport applications, the most used models are [92]:


The model of Gagge consists of a two-nodes model of the human body, representing the core and the shell, while the model of Stolwijk is a four-node model representing trunk, arms, hands, legs, and feet (Figure 6).

**Figure 6.** Human thermal physiological models: (**a**) Gagge's model; (**b**) Stolwijk's model [92].

The model of Gagge is used for most of the activities, while the model of Stolwijk is generally used only for the description of a wet swimmer coming out from the water. These models define the relation between individual parameters such as the metabolic rate and the clothing insulation with the environmental parameters, usually described by air temperature, relative humidity, mean radiant temperature, and air velocity.

For swimmers, thermal comfort depends on the time, due to the heat loss occurring because of the evaporation of the water on the skin, as swimmers are subjected to a drying process, which can be divided in two intervals [92].


In particular, the adaptation of the model of Stolwijk to the wet swimmer (during interval I) includes the addition of an evaporative term [92]

$$\mathbf{E(I) = (p\_{skin}(I) - p\_a) \ 2.2 \ h\_c(I) \ (10 \text{ v}\_a)^{1/2} \ f\_{\text{pcl}}(I) \ \mathbf{S(I)} \tag{14}$$

where E(I) is the evaporative heat loss of segment I of a wet body (W), pskin is the saturated water vapor pressure at the skin of segment I (Pa), pa is the water vapor partial pressure of the air (Pa), hc is the convective heat transfer coefficient (W/m2K), va is the air velocity (m/s), fpcl(I) is the Nishi's permeation efficiency factor of segment I and S(I) (m2) is the surface area of segment I.

For the calculation, usually the segment I is assumed equal to the nude part of the swimmer's body. The drying process is a transient condition, therefore the evaporative heat loss starts from an initial amount and ends when E(I) = 0 and the skin surface is dry. If the condition is when the drying process is just started (swimmer gets out of the pool), the pskin can be calculated for a temperature equal to the pool temperature [18]. For the dry swimmer (Interval II) the evaporative term should not be considered [92].

#### *4.2. Predictive Indices used to Assess Thermal Comfort*

In moderate climates, the thermal environment is defined through six basic parameters: four environmental (air temperature ta ( ◦C), relative humidity RH (%), mean radiant temperature tr ( ◦C), and air velocity va (m/s)) and two individual (clothing insulation Icl (clo) and metabolic rate M (Met)). It is the interaction of these factors which determines the thermal sensation of humans [23]. However, considering sport facilities, the conditions cannot be considered 'standard', as the metabolic activity can

be very high and clothing insulation may vary according to the sport apparel worn and to the increased air velocity due to the body movement. For this reason, in these environments the correct estimation of the individual parameters has a fundamental role in the prediction of the thermal sensation of the athletes.

#### 4.2.1. Fanger's Indices PMV and PPD

Most of the studies were based on the calculation of Fanger's indices, predicted mean vote (PMV) and predicted percentage of dissatisfied (PPD), derived from field measurements [14,16,17,19,22] or from simulations obtained through computational fluid dynamics (CFD) models [86,88].

PMV index is defined as the vote of an average individual regarding the thermal environment and it is a function of the six basic parameters, shown in Table 7. The calculation of this index requires an iterative method; therefore, it is generally performed through software, or directly by data loggers. The purpose of Fanger's indices is to correlate environmental and individual parameters to the subjective feeling of humans. Thus, Fanger proposed an experiment on 1296 individuals who, after remaining in a thermal chamber, had to give an answer regarding the thermal environment on a seven-point sensation scale, defined by ASHRAE (from -3 cold to +3 hot). Based on this survey, Fanger proposed the equation [93]

$$\text{PMV} = \left(0.303 \text{ e}^{-0.036M} + 0.028\right) \text{S} \tag{15}$$

where M is the metabolic rate and S is the heat storage.


PPD index is defined as the predicted percentage of dissatisfied with regard to a certain environment, considering dissatisfied a person who, subjected to a certain thermal environment, express a rating of +3, +2, −2, or −3 on the thermal sensation scale. The relation between the PMV and PPD is [93]

$$\text{PPD} = 100 - 95 \text{ e}^{0.03353 \text{ PMV}^\*4} + 0.2179 \text{ PMV}^\*2 \tag{16}$$

Note that the condition in which every subject is satisfied does not exist, and the minimum value of PPD is 5%.

However, the PMV method presents some limits, as indicated in the UNI EN ISO 7730 [70]. In fact, this method is applicable only when the environment can be defined moderate (PMV is less than 2 in absolute value) and when the six basic parameters stay within the limits shown in Table 8. This leads to the consideration that this method can present several problematics in the application to sport facilities, as the metabolic rate often exceeds the value of 4 Met.


**Table 8.** Range of applicability of environmental and individual parameters for the calculation of PMV [70].

4.2.2. Fanger's Indices PMV Corrected for Warm and Humid Environments (ePMV)

In some other cases [18,20] the PMV index was calculated considering the correction for non-air-conditioned buildings in warm climates, provided by Fanger and Toftum [94].

This model introduces the expectancy factor (e) shown in Table 9 that should be multiplied for the PMV in order to obtain the real thermal sensation of the athletes. This index is used to describe the perception of non-conditioned-buildings' occupants, as they may feel sensations of warmth less severe than the one predicted by PMV, due to their low expectations and factors related to metabolic activity [94].

**Table 9.** Expectation factor (e) for non-air-conditioned buildings in warm climates [18].


In particular, the ePMV can be calculated as

$$\text{ePMV} = \text{e PMV} \tag{17}$$

In the research of Revel and Arnesano [18,20], the expectancy factor was considered equal to 0.7. Furthermore, for the evaluation of the PMV in the swimming pool, the condition of the swimmer just coming out from water was considered and the evaporative term described by [14] has been added for the calculation of the PMV.

#### 4.2.3. Adaptive Comfort Model

The adaptive thermal comfort model has been applied for the parametric modeling used to predict and control of thermal comfort in a university multisport facility [87] and for spectators of a sport hall within an aquatic center [16], on the basis of the model of de Dear and Brager. In this model, it is taken into consideration that in general people in warm climate zones prefer warmer indoor temperatures than others living in cold climates [95]. This study consists of a statistical analysis which showed that occupants in naturally ventilated buildings present a wider tolerance with regard to the range of temperatures that can be recorded indoors.

#### 4.2.4. Operative Temperature

The operative temperature has been calculated for the assessment of the thermal environment in an indoor sport hall during summer period [14]. The operative temperature is often used for the assessment of the thermal environment, even if it is not considered in the six basic parameters and it depends on radiative and convective exchanges. It can be calculated as [55]

$$\mathbf{t}\_{\rm c} = \frac{\mathbf{h}\_{\rm r} \cdot \mathbf{t}\_{\rm r} + \mathbf{h}\_{\rm c} \cdot \mathbf{t}\_{\rm a}}{\mathbf{h}\_{\rm r} + \mathbf{h}\_{\rm c}} \tag{18}$$

where:

hc = unitary convective conductance (W/m2 K)

hr = unitary radiative conductance (W/m2 K)

ta = air temperature (◦C)

tr = mean radiant temperature (◦C)

Since there are some difficulties in evaluating the operative temperature with this equation, two simplified expressions for its calculation exist. The first provides a value of the operative temperature dependent on the relative air velocity by means of a coefficient A,

$$\mathbf{t\_{0}} = \mathbf{A} \ \mathbf{t\_{0}} + (1 - \mathbf{A}) \ \mathbf{t\_{0}} \tag{19}$$

where A = 0.5 when the relative air velocity is lower than 0.2 m/s, A = 0.6 when the relative air velocity is between 0.2 and 0.6 m/s, and A = 0.7 when the relative air velocity is between 0.6 and 1.0 m/s.

The second expression is an arithmetic mean of values of the two temperatures from which the operative temperature depends

$$\mathbf{t}\_{\rm O} = (\mathbf{t}\_{\rm a} + \mathbf{t}\_{\rm r})/2\tag{20}$$

4.2.5. Humidex

The humidity index (humidex) has been used to assess thermal comfort in a swimming pool [15]. This index is a dimensionless number which allows the evaluation of air temperature and humidity on an average person. It has been proposed for the evaluation of environments which presented high humidity as an alternative to the Fanger's indices. The value of humidex is given by [93]

$$\text{Humidex} = \text{t}\_{\text{a}} + [5.555 \, (\text{Pa} - 1.013)] \tag{21}$$

where:

ta = air temperature (◦C) pa = partial vapor pressure (kPa)

Humidex identifies 4 different thermal levels, from the comfort level (level 0) to the definition of possible health risks (level 4).

#### *4.3. Subjective Judgements Used to Assess Thermal Comfort*

Even if this aspect is often underestimated, the perception of a thermal environment is strongly related to the psychological conditions. For this reason, most of the researchers in their studies assessed the thermal sensation of the athletes with the use of questionnaires [18,19,21,89]. In particular, with regard to the subjective perception of the thermal environment, the research of Revel and Arnesano [18] compared the predictive models to the subjective responses in order to determine the impact of high metabolic rates and sport garments on systematic errors in the prediction of Fanger's indices.

For the design of the questionnaires, UNI EN ISO 10551 Standard [96] regulates the subjective evaluation of the thermal environment, which consists of judgement scales regarding perception, comfort, and thermal preference and, in some cases, personal acceptability and tolerance. In particular, the scale of perception in moderate environments is represented on a seven-point scale showing the thermal sensation vote (TSV), which can be compared to the PMV. Furthermore, the evaluative scale shows the feeling of comfort of the subjects on a unipolar scale from 0 to 4, in which points 3 and 4 are characterized by an increasing level of discomfort and can be considered dissatisfied. For this reason, this scale can be compared to Fanger's PPD.

#### *4.4. Thermal Environment and Performance: Correlation between PMV and RPE*

The perception of the thermal environment has a great influence on the performance, which is related directly to the kind of activity that has to be carried out in terms of concentration, physiological effort, etc. Several studies have been performed on the problems related to the heat stress of the athletes in hot and humid environments, but little knowledge is available in moderate environments. In fact, even if in these environments the safety of the athletes is not usually compromised, their performance may be affected by certain ambient parameters. For this reason, in sport facilities, the impact of the thermal environment on the athletes has been considered, as better conditions could improve the efficiency of a training session, or even the performance in a competition. In particular, studies were carried out in order to correlate the Fanger's index PMV to the rate of perceived exertion (RPE), in order to find an association between the thermal and the physiological responses of athletes during exercise [22]. Furthermore, Zhai et al. [21] studied the effect of air movement for comfort during exercise at different levels of metabolic rate and the relation between the thermal sensation and the perceived exertion. In these researches, physiological characteristics such as the oxygen consumption, skin, and core temperatures were also detected. It resulted that PMV is related to RPE, thus increasing the condition of thermal comfort in sport facilities may have positive results also on the performance of the athletes.

#### **5. Thermal Comfort Assessment in Practice**

The assessment of thermal comfort in sport facilities presents some difficulties due to the complexity of these environments. In fact, often sport halls are multifunctional buildings, used for the practice of several sports. Furthermore, in sport facilities the activities carried out are not stationary, thus the six basic parameters cannot be assessed as in other environments such as offices, school buildings, etc. In this section, a review of the methodologies used to assess thermal comfort in sport facilities is reported.

#### *5.1. Monitoring Duration*

The monitoring duration was 2 days at minimum and 5 months at maximum. Data were recorded with a minimum of 15 seconds to a maximum of 15 min. The time of acquisitions varies largely in relation to the kind of environment investigated. When the conditions change rapidly, the acquisition time must be shorter (every 15 sec–1 min), otherwise data can be recorded every 15 min. Any standardized procedure was revealed by the review of the existing research on thermal comfort in sport facilities and the measurements were carried out in relation to the problematics of the case of study.

#### *5.2. Assessment of the Individual Parameters*

#### 5.2.1. Metabolic Rate

The metabolic rate can be determined by UNI EN ISO 8996 standard [53], which contains the data to calculate it with tables provided for different activity levels. However, more accurate estimation can be provided by studies concerning the level of metabolic activity of the sport considered [54]. For varying metabolic rates, standard UNI EN ISO 7730, suggests a time-weighted average that should be estimated during the previous 1 h of activity [70].

In the research of Revel and Arnesano [18], three phases were examined: transitory phase, steady state, and recovery state. These phases should be considered in the calculation of PMV, as they imply exercise levels. In fact, also the intensity of the exercise is fundamental for the metabolic production. In particular, sedentary activities are characterized by values around 1.0–1.5 Met, light intensity 1.6–2.9 Met, moderate intensity 3–5.9 Met, and vigorous intensity > 6 Met [54]. The correct evaluation of the metabolic rate is fundamental for the calculation of PMV and PPD, as mistakes in the assessment may lead to uncertainties. For this reason, researchers should focus on the determination of this parameter, in order to perform a correct evaluation of the thermal sensations.

#### 5.2.2. Clothing Insulation

Standard UNI EN ISO 9920 [27] provides a procedure for the evaluation of the clothing insulation with tables reporting Icl values (m<sup>2</sup> K/W) for several clothing types. However, often sport garments are not included in this standard, thus specific research on the values of their insulation have been used by researchers, in order to provide a more precise estimation.

In sport facilities the clothing worn can be the same for all the athletes (in case in which a particular uniform is worn) or, more commonly, the garments may change according to the personal preference. For this reason, researchers calculated values of PMV and PPD for different clothing ensembles [17] or they were asking in questionnaires provided to users which were the garments worn during sport activity at the time of the test [18]. In the cases in which the movement of the human body was relevant and it modified the thermal insulation of the clothing due to the pumping effect, the correction proposed by UNI EN ISO 9920 was considered [27]. This correction is a function of air velocity and metabolic rate and it involves a decrease of the thermal insulation of the garments, which allows a greater heat transfer.

#### *5.3. Measurement of the Environmental Parameters*

The measurement of the four environmental parameters is also fundamental for the determination of the Fanger's indices. In buildings such as offices, where stationary activities are carried out, the probes are usually located close to the workstation or, in any case, in the proximity of the locations occupied by building users [55]. In sport facilities, athletes do not occupy a fixed position, thus the procedure for performing the measurements is more complex. However, in the review of the existing research, it resulted that the location of the probes in the sport facilities was standardized, as they were situated at a height varying from 0.6 m for the sitting position to 1.1–1.7 m for the standing position, usually in the center of the hall or, in the case of the swimming pool, close to the water [20].

#### **6. Conclusions**

In sport facilities, ensuring thermal comfort is particularly relevant, as it may affect the performance and health of the athletes. Thermal environments can be seen as a combination of physical, physiological, and psychological factors, and the interaction between these three aspects determines the thermal sensation of humans. In particular, in spaces in which physical activity is carried out, the physiological component is particularly relevant, as the metabolic rate is high and therefore the body produces a consistent amount of heat, which must be dispersed in the environment.

In order to predict the thermal sensation of the athletes performing in indoor sport facilities, most researchers focused on the calculation of Fanger's indices PMV and PPD. However, even if a correlation between the PMV corrected for warm and humid environments and the real sensation determined through questionnaires was found, the high metabolic rate occurring during sport practice may lead to an overestimation of the thermal sensation of the athletes. Furthermore, since sport facilities are multifunctional buildings in which several activities are carried out, the difficulties that have to be faced also concern the determination of a thermal environment which is comfortable for all the occupants, from the athletes to the spectators.

From the literature review, it results that only little knowledge is available on the determination of thermal comfort in indoor sport facilities and on the standardization of a measurement protocol to be applied in these spaces. Moreover, there is a lack in standards concerning the environmental parameters that should be maintained in sport halls. For this reason, further research should be developed on this topic, as performing in a comfortable environment may improve the performance of athletes and ensure healthy and pleasant conditions.

**Author Contributions:** All the authors contributed in equal parts to the research activity and to the paper writing. **Funding:** This research received no external funding.

**Conflicts of Interest:** No conflict of interest or personal relationships that could have appeared to influence the work reported in this paper.

#### **References**


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