*Review* **Biometric Data as Real-Time Measure of Physiological Reactions to Environmental Stimuli in the Built Environment**

**Sandra G. L. Persiani \* , Bilge Kobas , Sebastian Clark Koth and Thomas Auer**

Chair of Building Technology and Climate Responsive Design, Department of Architecture, Technical University of Munich, 80333 München, Germany; bilge.kobas@tum.de (B.K.); sebastian.koth@tum.de (S.C.K.); thomas.auer@tum.de (T.A.)

**\*** Correspondence: sandra.persiani@tum.de

**Abstract:** The physiological and cognitive effects of environmental stimuli from the built environment on humans have been studied for more than a century, over short time frames in terms of comfort, and over long-time frames in terms of health and wellbeing. The strong interdependence of objective and subjective factors in these fields of study has traditionally involved the necessity to rely on a number of qualitative sources of information, as self-report variables, which however, raise criticisms concerning their reliability and precision. Recent advancements in sensing technology and data processing methodologies have strongly contributed towards a renewed interest in biometric data as a potential high-precision tool to study the physiological effects of selected stimuli on humans using more objective and real-time measures. Within this context, this review reports on a broader spectrum of available and advanced biosensing techniques used in the fields of building engineering, human physiology, neurology, and psychology. The interaction and interdependence between (i) indoor environmental parameters and (ii) biosignals identifying human physiological response to the environmental stressors are systematically explored. Online databases ScienceDirect, Scopus, MDPI and ResearchGate were scanned to gather all relevant publications in the last 20 years, identifying and listing tools and methods of biometric data collection, assessing the potentials and drawbacks of the most relevant techniques. The review aims to support the introduction of biomedical signals as a tool for understanding the physiological aspects of indoor comfort in the view of achieving an improved balance between human resilience and building resilience, addressing human indoor health as well as energetic and environmental building performance.

**Keywords:** biometric data; biosignals; non-intrusive sensing; physiological metrics; environmental stimuli; stress detection; health; comfort

#### **1. Introduction**

Urban living is on the rise. The majority of the population worldwide lives in urban areas and expectations are that the number of global urban inhabitants will grow up to two thirds in the next 30 years [1]. At the same time, research shows that people also spend a great majority of their time in closed environments made by humans; over the past few decades people on average spent between 90% and 98% of their time indoors depending on season and holiday time [2–5]. These numbers can also be expected to rise further during crisis events forcing people indoors—be it heat waves, hurricanes, or global pandemics.

The progressive detachment of humans from the natural world has proven to have a series of negative impacts on users ranging from low productivity [6], disruption of the circadian rhythm [7,8] and the hormonal system [9], resulting in diseases such as sleep disorders, immune system disorders, macular degeneration, cardiovascular diseases, diabetes and osteoporosis to cancer [10]. Furthermore, as we increasingly become an "indoor species", the indoor built environment is known to pose serious health hazard risks, commonly known as sick building syndrome (SBS) or "building-related illnesses" (BRI) [11,12]. They derive from a lack of good design and proper maintenance and the use

**Citation:** Persiani, S.G.L.; Kobas, B.; Koth, S.C.; Auer, T. Biometric Data as Real-Time Measure of Physiological Reactions to Environmental Stimuli in the Built Environment. *Energies* **2021**, *14*, 232. https://doi.org/ 10.3390/en14010232

Received: 1 December 2020 Accepted: 28 December 2020 Published: 4 January 2021

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**Copyright:** © 2021 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 (https:// creativecommons.org/licenses/by/ 4.0/).

of new hazardous chemicals in building materials as well as in furnishings and consumer products [9]. Being largely undocumented, exposure levels to the chemicals in everyday life should seemingly increase with people spending more time indoors and air exchange rates decreasing as buildings are made more airtight to improve energy efficiency [9,12].

Achieving healthier indoor conditions also becomes relevant from the perspective of energy and environmental building efficiency, if the bigger picture of overall sustainability is considered. Not only have researchers claimed that improved indoor conditions affecting user productivity can end up with massive savings on a company's operational costs [13,14], but the overall energy use has been clearly described as a consequence of trying to attain comfort (in terms of homeostasis) [15].

Therefore, we urgently need to improve our understanding of short- and long-term impacts of the indoor environments on us, in order to not only design energy and resource efficient spaces, but also ones that improve the health and wellbeing of people [16].

#### *1.1. Measuring Human Wellbeing and Health in Indoor Environments*

Wellbeing and health are naturally related concepts. While these can be considered to consistently impact each other, they are neither synonyms nor consequent [17]. The World Health Organization (WHO) defines health as a "state of complete physical, mental and social wellbeing", in other words, a state that goes beyond the mere absence of disease [18]. While "wellbeing" is an easy concept to grasp when contextualized, it remains hard to define and predict [19]. It is widely used across a great number of disciplines, setting focus on different aspects such as physical health (absence of disease) and comfort, psychological aspects (individual internal generation of meaning and sense of self), ethical aspects, social aspects (comparison with others as a means of assessing one's own abilities and functioning), economical aspects (standard of living, the real opportunities available to individuals), etc. [19,20]. In many cases, wellbeing, wellness [21] or also subjective wellbeing (SWB) [20] are interchangeably used and measured in terms of "happiness" [19,20,22] or "life satisfaction" [20]. These concepts are not only very broad but are also to a big part based on subjective judgments of satisfaction, as well as individual aims and values, which can change over time and between contexts. This creates critical challenges in terms of measurement and comparison due to differences in type and magnitudes of the chosen parameters and experimental conditions along with notorious problems of information bias even in studies with good reliability and validity [22].

Needing more objective parameters to evaluate the quality and efficacy of wellbeing conditions and interventions, SWB has been further broken down into concepts that include more measurable and objective factors [20,22].

In general, the human reactions that can be observed in response to environmental agents can be classified into three kinds: (i) behavioral, (ii) physiological and (iii) psychological [23]. Behavioral aspects are coordinated responses of individuals or groups to internal and/or external stimuli [24] including objectively observable activities and nonconscious processes (habits) [25]. These actions, reactions, or mannerisms (habitual gestures or way of speaking or behaving [26]) mostly refer—in the context of building physics—to physical (body) actions. Physiological aspects refer to the autonomic responses of the bodily parts of an organism to a stimulus, whether as a form of homeostasis, withstanding changes in environmental conditions that are outside their optimal range [27], or to trigger a behavioral reaction in response to an immediate threat [28]. The rising availability of sensing technologies and data processing methodologies has turned biometric sensors into interesting high-precision tools to study the physiological effects of environmental stimuli on humans [29]. Psychological aspects are functional processes, operations, and changes that relate to the mental and emotional state of an individual (or a group of individuals in the case of group dynamics). Depending on what is measured and, on the methods used, different combinations of these three aspects are taken into account.

Which parameters and methods are appropriate to reliably measure the effect of environmental conditions on users?

The answer to the question is inevitably complex as it depends, among many factors, on the timeframe taken into account and the overall aim of the study. While health and psychologically oriented studies tend to focus on measuring outcomes as individual stress or satisfaction [30–34], economically oriented research tends to use more performancebased criteria focusing on productivity [35,36].

For what concerns the built environment, the significance of the environmental context and conditions in determining human health has mainly been the object of environmental psychology studies [37,38], healthcare environment design [15,38–41], and studies on sick building syndrome [15]. Research in the field of building physics on the other hand has largely focused on indoor comfort [15,16,30,42–51] and user satisfaction [30,32] as a measure of the indoor environmental conditions, and in some cases also on productivity and performance [2,14,35,52–54]. These methods follow, at heart, radically different visions of human wellbeing and/or health, and can be grouped into three major categories:


#### *1.2. Self-Selection Metrics*

*Self-selection* refers to the act of individually opting "in or out of something (such as a group, activity, or category) in accordance with one's personality, interests, etc." [14]. The term is used in economic and social statistics referring to a type of selection bias that can arise when following rules of non-probability sampling, as decisions deriving from self-selection are the object of study [55]. By distorting the selection rules, self-selection makes determination of causation more difficult, resulting in influencing the data.

The term "self-selection" is generally not used in the field of building physics, although three main methods used to measure the quality of indoor environments (preference, satisfaction, and comfort) are overtly known to be highly subjective [22,50,56–58]. Experiments in the field of architecture largely rely on soft data assessed by questioning test subjects about their preferences, which is the case in a great majority of thermal comfort studies [6,16,30,42,50,59–62].

#### 1.2.1. Preference, Acceptance and Satisfaction

Preference involves a choice between two or more alternatives. In preference testing, users or consumers are typically given a choice and asked to indicate their most liked option [63]. Acceptance (or liking) involves rating a specific option or aspect on a scale and can also be achieved without the need of comparing the solution with any other one [63]. What fundamentally connects preference and acceptance as measures is that none of these concepts reflect an ideal solution (the best possible), but at most an optimized solution. Both terms are largely used in business-oriented fields for consumer testing. Research in the context of the built environment also employs these terms [30] to estimate the quality of indoor environments.

Satisfaction is a more complex type of testing as it involves, from the side of the test subject, the rating of a specific option against one's individual scale of values and expectations. As compared to preference and acceptance, satisfaction does in fact reflect a positive assessment. Despite the innate complexity of the valuation method, satisfaction is a well-established metric for assessing quality in the indoor environments [32,57,64]. In building engineering, an indoor environment is generally considered acceptable when 80% of the occupants are satisfied with the conditions [58]. Occupants' environmental satisfaction is directly related to the amount of perceived comfort [32,64], and both of these aspects sequentially impact user performance [14].

All three concepts are often, and sometimes interchangeably, used in indoor environmental research to describe user feedback and assess environmental conditions. While part

of the assessment truly depends on the physical conditions of the context, a large part of the estimation is determined by individual psychological conditions.

#### 1.2.2. Comfort

*Comfort* is defined as the "absence of unpleasant sensations" or as described within the most widespread definition in the field of building physics "the condition of mind that expresses satisfaction with the (thermal) environment and is assessed by *subjective* evaluation" [58]. It is therefore, by definition, a lack of discomfort: a neutral state with no stress. From a biological perspective, comfort can be effectively compared to the maintenance of homeostasis, indicating the absence of environmental stressors [15]. Comfort is broadly recognized as a multidimensional and subjective construct that varies across contexts [15] and depends on functional and environmental factors, on personal health and mood and is recognized as being highly subjective [14,15,47,65].

A large majority of comfort studies are based on the unvoiced assumption that physical health is a direct consequence of comfort. The gap existing between the definition of indoor comfort conditions and long-term health conditions however becomes apparent when looking at studies showing correlations between time spent in thermoneutral environments and the likeness of developing obesity [66–69]. Similarly, exposure to conditions outside the thermoneutral zone have shown to reduce the susceptibility to developing type 2 diabetes [70].

Measuring comfort is a difficult task. Comfort studies rely largely on self-reports and other types of user-feedback methods [6,16,30,42,50,59–62]. Though subjective measurement remains a major research method in many disciplines, the reliability of the results is contested by many authors [56]. If on one hand self-reports allow taking into account the mental state and mood of users, the data also involve a large risk of information bias [22,71]. The feedback is not only strongly influenced by the individual character of the perceived conditions, which can be minimized if analyzed over larger numbers of individuals, but psychological components are also known to consistently alter users' perceptions and thermal expectations of an environment [50,56]. Moreover, the data typically lack precision in timing, are generally not scalable and are usually difficult to reproduce [56]. Moreover, measuring comfort involves the collection of processed and combined data, such as physiological factors (thermoregulation), potentially random or non-repeating variables such as climatic behavior (subjects turn on the desk fan, wear another piece of clothing, etc.), and psychological or other highly individual factors (emotional stress levels, hormonal levels, etc.). Although current practice statistically normalizes individual differences in the datasets by collecting data on a large scale, it also involves significant physiological aspects not being easily parsed from the datasets.

#### 1.2.3. Considerations on Self-Selection Metrics

Investigating the *perceived* indoor environmental conditions is on one hand an important task, as it takes the objective physical measures of environmental data into account, as well as the psychological mindset of the occupants [72]. However, from a health perspective, the importance of users' perception is relative for two main reasons:


known, the choice between an immediate gratification (e.g., smoking) and a delayed gratification (health) is in psychological terms not always obvious [73].

In all self-selection metrics described, issues concerning potential systematic data bias have been raised. Surveys, questionnaires, and other self-reporting systems can also be argued as to discard neutrality as the subjects are made temporarily consciously aware of their surroundings when they are questioned about them. Additionally, these highly individual pieces of information pose problems with replicability, scalability and comparability of the said data.

#### *1.3. Performance-Based Metrics*

*Performance-based* metrics are most often aimed at measuring the efficiency of a system in economic terms, whether the outcome is expressed as environmental, financial or process excellence. Such metrics are effective to express usability and to inform key decisions [74] as they reduce complex measurements and result in a single value that can be tracked, managed, and improved. While complexity is reduced, looking only at the outcome of a network of interacting factors, it is important to keep in mind that these shortcuts can also become misleading when used for process improvement.

In the context of the built environment, performance-based metrics are used to express functionality in domains such as energy performance, indoor environmental quality, environmental impact, capital, and operating costs [75–77]. Few studies have focused on performance metrics in the occupant domain, and even those mostly focus on the built environment's qualities rather than on the effect on users [75].

Typical user-based performance metrics measure task success (binary success or levelbased success), time on task (completion of a task within a time limit), error-based measure (single or multiple error opportunities), efficiency (number of actions required to complete a task, ratio of task success rate to the average time per task) and learnability (how any efficiency metric changes over time) [2,14,33,35,36,38,74,78,79].

#### 1.3.1. Productivity

A number of studies have attempted to measure the effect of indoor environmental conditions by evaluating user performance in terms of *productivity*, mostly focusing on the thermal aspect [2,14,51,78] and on indoor air quality [14,53,78]. The underlying concept is that as environmental conditions exceed the range of comfort, the human body adapts to sustain the level of task performance. This is achieved through homeostatic adaptation on one hand and through psychological adaptive behavior (as attentional focus) on the other [51]. If the environmental conditions exceed the body's maximum range of adaptability, the attentional resources are depleted, and human performance ultimately deteriorates [51]. Assessing productivity can be achieved either using (i) subjective approaches, with self-assessing methods to rate perceived performance over a specific period of time [2]; (ii) quantifying productivity by defining a context- and content-specific ratio of output to input (ratio of company turnover to employee cost [80], individual/team performance [35,36,81], etc.) and gathering data in a real environment [14]; (iii) evaluating performance based on specifically designed tests (neurobehavioral tasks such as logical/comprehension/numerical/visual/memory etc. [82] or perceptual motor tasks [83]). The main drawbacks of assessing indoor environmental conditions by measuring productivity come from the environmental effects not being directly reflected on the subjects' task performance [84], due to partial adaptation to the stressing agents [51] and individual motivation [85], which can offset the effects on task performance. In fact, productivity is known to be influenced by many factors, among which are personal, social, organizational, and environmental factors [14].

#### 1.3.2. Attention and Distraction

Another performance measure used is *selective attention* and its counterpart, distraction. To overcome its limited processing resources and reduce the amount of sensory information

it needs to comprehend [86], the brain uses various filtering mechanisms. It therefore constantly oscillates between states of deep focus—as we momentarily disengage from the real world when we are highly concentrated on a task [87]—and awareness of our surroundings when we are distracted by environmental stimuli [86]. The oscillations between bursts of attention can therefore be closely linked to our behavior [88], providing indications on the effects the surrounding environments have on us. Specifically, increased cognitive engagement (attention) is known to produce decreased sensitivity to visual events [87] with slower reaction times, decreased situation awareness [89], and specific eye movements such as spontaneously closing the eyes or looking away [90]. The neural oscillations leading to selective attention, alternating periods of either heightened or diminished perceptual sensitivity, have been studied through human behavioral studies [88,91,92], but also using physiological measures as electroencephalography (EEG) [88] and oculomotor capture [87]. Researching the impact of selective attention can be a complex task, as can be seen by the multiple attempts to collect and interpret data from different sources:


#### 1.3.3. Mental Workload

Similarly to selective attention, *mental workload* or mental fatigue, defined as "the mental resources devoted to the tasks of an individual" [103], is used to measure a lack in performance typically characterized by higher error rates, decreasing efficiency and alertness, and effort disinclination [104], resulting in some cases also in detrimental health effects on long timeframes [105,106]. While measurements of productivity are limited and offset by individual motivation and ability to maintain the performance, mental workload can be assessed using physiological measures, such as electroencephalography (EEG) on subjects while being asked to perform tasks specifically designed to activate typical cognitive functions [2]. Other commonly used measures of physiological metrics include event-related potentials, magnetoencephalography, positron emission tomography, electrooculograms, cardiovascular measures, pupillometry, respiratory measures, and electrodermal measures [107].

#### 1.3.4. Considerations on Performance-Based Metrics

Summarizing the reviewed types of performance-based metrics, only selective attention and mental workload appear to enable an objective measurement of the effect of the surrounding environment on users through the use of physiological measurements. These systems measuring cortical activity are on one hand accurate, providing a good option as a quantitative approach but present some intrinsic limitations: (i) datasets are very complex to read, and in some cases (as selective attention) do not provide a specific EEG signature; (ii) datasets can potentially be affected by many other factors [2]; (iii) the potentially high number of false positives due to interference from other body functions and movements. Reliability of the results can be theoretically achieved by adopting a multimodal approach [89], combining two or more physiological detection methods.

As far as this review could find, these methods are commonly used in the field of neuroscience [108], psychiatry [109], computer science and biomedical informatics [110], as well as architecture, computing and engineering [89], but very few attempts have been undertaken in the field of building physics [2,111].

As discussed in the beginning of the paper, another aspect that part of building physics studies focuses on, apart from comfort (or *sensation* and *preference*), is mental workload and productivity. While both terms have a similar individual complexity, they also have a

similar timeframe: both comfort and productivity are momentary states. However, these are also some of the most important factors in building operations and have a direct impact on the economy and use of resources. This argument is significant on a strategic level. While comfort studies prioritize optimizing building energy use and user satisfaction, and studies on productivity bring a more direct economic impact; both constellations forgo the health implications as they are not visible at the same timescales.

#### *1.4. Physiological Metrics*

Physiological aspects refer to the autonomic responses of the bodily parts of an organism to a stimulus, to withstand changes in environmental conditions that are outside their optimal range [27], or to trigger a behavioral reaction in response to a threat [28].

#### 1.4.1. Discomfort

As previously discussed, comfort perception is the main key measure used in building physics to assess the quality of indoor environments, although the concept is known to be strongly impacted by psychological factors and the data are potentially biased. The sensation of *discomfort* on the other hand is the complementary entity of comfort [112] and is more broadly researched in fields such as ergonomics [112,113], medicine [114–116] and psychology [33,117,118]. Discomfort is described as a mental or physical uneasiness or annoyance [119], resulting in a natural response of avoidance or reduction in the source of the discomfort [114]. While in many studies physical discomfort is used as a synonym of pain [33,116], not every discomfort can be attributed to pain [114,115]. In most research, discomfort is identified and measured using self-report [112], assessing the severity, frequency, and duration of work-related body-part discomfort [113,116], or the observation of physiological and behavioral outcomes [114,116]. Generally, literature agrees on the subjective nature of the sensation of discomfort [114,116–118]. In physiological terms, there are no receptors to measure comfort (which is defined as lack of discomfort), while discomfort does in fact leave a mark.

#### 1.4.2. Stress

*Stress* is the prototypical response to an internal or external event, force, or condition (the stressor), triggering a cascade of processes to help the body to adapt [25]. The effects are behavioral, physiological, and psychological [120], testifying how the stress experience relates both to the objective perception and the subjective evaluation of an event [56]. Physiological stress response is largely involuntary and is mediated by the autonomic nervous system (ANS) [56] as neurotransmitters and hormones signal action to the body [121]. Examples of the physiological stress response are palpitations, sweating, dry mouth, shortness of breath, fidgeting, accelerated speech, augmentation of negative emotions (if already being experienced), and longer duration of stress fatigue [25]. Although the concept is often negatively connotated, under certain circumstances, stress can also have positive adaptive effects such as arousal [56,122] and enhancing immune function (e.g., preparing the immune system for challenges such as wounds or infection) [121]. The impact on health and performance has also been shown to vary depending on the person's individual mindset regarding the stress conditions [123]. Damaging, long-term consequences of stress are essentially encountered as the reaction becomes a chronic condition [29,124–127]. Stress has been suggested to have four main components impacting health [128]: exposure, reactivity, recovery, and restoration. (i) The number of stressors experienced (exposure); (ii) the strength of the individual physiological reaction (reactivity); (iii) time of recovery from the stress reaction (recovery); (iv) healing processes of the organism (restoration), which might be hindered by the stressful condition.

In other words, stress is the physiological reaction of the body to a necessity of adaptation, triggering the body's homeostatic functions to regulate (temperature, heart activity, blood pressure, respiration, and glucose levels) [56,129,130]. As long as the range of adaptation required is within the capacity of the organism [131], the dominant perception

is of comfort. If the stimuli exceed the capacity of adaptation, the resulting sensation is of discomfort. Stress can therefore become an interesting measure in the field of building physics as a source indicator for physiological change in conditions. Especially as, contrary to the case of mental workload, stress has many known biophysical signatures, allowing for better data precision and reliability [89].

## 1.4.3. Considerations on Physiological Metrics

Physiological metrics appear to have a good potential to sustain the current mainstream data collection methods in the field of building engineering, potentially offering reliable and objective methods to measure environmental stressors. Specifically, recent advancements in sensing technology and time-sensitive biomedical data acquisition are opening up new possibilities to use biomedical signals as indicators of quality. The promise of parsing physiologically relevant data is that the impacts of additional variables, such as adaptive behavior, tolerance, psychology, exposure time, can be studied against a constant, in a situation where almost all other parameters vary.

#### *1.5. Research Gap*

More objective means are needed in the field of building physics to measure the effects of indoor environments on users as the mainstream parameters of indoor environmental quality, comfort, and discomfort are widely accepted as subjective sensations and data collection methods are prone to be biased. Following a review of methods to measure human wellbeing and health, the use of physiological signals to detect stress is seen as a promising alternative.

The use of these methods is however relatively new to the field of building and climate engineering, where solid theoretical foundations need to be established before being systematically used. While biosignal patterns are regularly researched for psychological, social, and mental stress conditions, comparatively few attempts were found to be made in the field of building physics to investigate the patterns of individual biosignal features triggered by stress factors originating from the built environment.

Within this context, this review reports on a broader spectrum of available and advanced biosensing techniques used in the fields of building engineering, human physiology, neurology, and psychology. The aim of this review is to offer a systematic and comprehensive insight on the current capacity to detect stress from the built environment using biosignal measures. Emphasis is put on the efficiency, robustness, and consistency of biosignals as a data source. Specifically, the paper will:


#### **2. Materials and Methods**

The paper explores the interaction and interdependence between (i) indoor environmental parameters affecting humans and (ii) human biosignals that respond to the environmental stressors by activating a physiological response. As such, the review was developed in subsequent phases of research and analysis. The topic is addressed from a holistic point of view, defining the relevant features through a qualitative and multidisciplinary research approach. State-of-art knowledge in the relevant fields was reviewed by

searching online available databases: ScienceDirect, Scopus, MDPI and ResearchGate and through the university libraries of the authors' home university.

A total of 246 sources were reviewed throughout this paper. A large majority of the publications, 82.5% of the total amount, were published after 2000, and 93.1% were published after 1990. Figure 1 shows a histogram based on publication years.

**Figure 1.** Histogram of publication years of sources.

Out of these 246 sources, journal papers seem to be the absolute majority when looking at the types of the publications. Figure 2 shows the breakdown of the entire bibliography based on the publication types.

**Figure 2.** Breakdown of publication types.

Due to the interdisciplinary nature of the research topic and aiming to achieve a broad overview of the methods used to measure human health and wellbeing, the research reviewed appears to be extremely broad and diverse. Table A1 categorizes the publications based on their authors' original research fields and Figure 3 illustrates their distribution. As can be seen in both Table A1 (please see Appendix A) and Figure 3, nearly half of the

reviewed material comes from medical fields (general medicine, neuroscience, biomedical engineering, biology, psychology) and 31% originates from fields related to built environments (architecture, building engineering, building physics). The third prominent discipline seems to be computer sciences with 11%, including relevant sensing and data acquisition technologies or data analytics.

**Figure 3.** Distribution of main research fields of authors.

#### *2.1. Methods for Measuring Human Wellbeing and Health in Indoor Environments*

For the first step, the available methods commonly used, according to literature, to measure human wellbeing and health in the indoor environment were briefly summarized. The databases were searched with the keywords "health, well-being, indoor environment, self-selection, assessment, questionnaire, survey, satisfaction, performance, comfort, productivity, attention, mental workload, fatigue, task performance, cognitive performance, physiological, discomfort, stress, biosignal".

For this part, 157 sources were reviewed in total: 10 books, book chapters and theses, 18 conference papers, 121 journal papers, 4 standards, guides or reports and 4 websites. The publication dates range from 1950 to 2021, with 130 of the publications dated after 2000.

The majority of the research came from the fields of built environment (28/157) and indoor environment (building physics and engineering, 24/157) as well as medicine (27/157). Table A2 shows the sources used in this chapter based on the authors' main research fields in further detail (please see Appendix A).

Table A3 provides a more specific grouping for the publications contributing to the review and discussion of definitions of human aspects. "Stress" comes forward as the most repeated term amongst these publications, mostly with medical origins (25 publications out of 38) and computer sciences and engineering background (12 out of 38), with no occurrences in built environment or building engineering originated publications. However, the term "comfort" is the most common in these two fields (with 23 out of 36), with only 6 from a medical background. Finally, "health" comes forward as a shared term, as it appears in the publications from a medical background 21 times out of 36, and 14 times in the sources with a built environment background.

#### *2.2. State of Art: Indoor Environmental Quality (IEQ) Parameters*

For the second step, indoor environmental parameters typically used in the field of building physics were summarized in order to give a structured overview of the aspects that are usually taken into consideration in the design and environmental regulation of indoor spaces. The databases were searched using different combinations of keywords, such as "indoor environmental quality, IEQ, air quality, IAQ, thermal IEQ, thermal comfort, visual IEQ, visual comfort, visual health, acoustic IEQ, acoustic comfort, acoustic health, indoor health, comfort, occupant comfort, user comfort". The literature available on IEQ parameters appears to be very broad as the concepts are well-established in the field of building physics. This is the reason why the literature search was focused on identifying relevant reviews, proceeding in a second step to find specific articles that focused on single aspects of interest.

As a result, 90 sources were reviewed in total, amongst which 50 of these sources were also used in the previous summary of measures for human health and wellbeing. The search includes recent as well as well-established sources on different indoor environmental condition theories between the years 1973 and 2021, and 75 of these were published after 2000. As can be seen in further detail in Table A4, the majority of publications (72 out of 90) are journal papers (please see Appendix A).

The majority of the reviewed material came from the field of building related research and building physics (in total 58 out of 90). Table A5 shows a catalogue of IEQ-related publications reviewed in this paper, based on their authors' main research fields.

Looking at the cross relation of human factors and IEQ parameters, it seems that "health" and "comfort" come forward as frequent terms used in reviewed IEQ research, followed by "productivity", "performance, mental workload". However, comparing Table A6 to Table A3, where the sources on human aspects were categorized based on their focus, it was revealed that the literature on "stress" was the most prominent. However, in Table A6, it becomes clear that research on stress in relation to IEQ is not equally prominent; with "health" (33/90) and "comfort" (25/90) having higher priorities amongst reviewed publications on IEQ, while "stress" being found as the focus point of a single publication in this category.

Looking at the distribution of subcategories of IEQ parameters, thermal IEQ research presents itself as most prevalent, with 26 publications out of 90, while acoustic IEQ research comes last with 9 publications. Table A7 provides an overview of sources based on their relative IEQ parameter focuses.

Aspects that are relevant in terms of human health and comfort or otherwise useful for further bridging with biosensing techniques were identified and summarized. More specifically, all IEQ parameters were discussed in terms of:


#### *2.3. Background Research on Physiological Signatures*

For the third step, an extensive investigation of published studies on available biosensing techniques, used to measure human stress response, was performed. Broad research criteria were initially adopted in order to take into consideration a larger pool of potential biosignals. Databases were therefore scanned using different combinations of a number of keywords, among which were "biosignals, biologic signals, biomedical signals, biosensors, neurophysiology, psychophysiology, electroencephalogram (EEG), electrocardiogram (ECG), heart rate (HR), electrodermal activity (EDA), galvanic skin response (GSR), physiology of comfort, physiology of stress, stress recognition, mental workload, stress hormones", and the research was expanded to the sources cited by the selected literature. The search was subsequently narrowed down to more recent publications, focusing on research and

technologies achieved within the last 20 years (with the exception of publications that were of specific relevance).

As a result, a total of 110 publications were reviewed; the majority (78/110) being journal papers. Table A8 shows the sources based on their publication types.

When checked against the authors' main research fields (Table A9), it was clear that medical fields take the lead, followed by computer sciences and engineering in sensing and data acquisition technologies. Amongst the reviewed sources, research from built environment or building physics seems to have weak direct correlation to physiologic or biologic methods in terms of human aspects (health, comfort, wellbeing) in the context of indoor environmental quality.

#### *2.4. Selection of Biosensing Techniques*

Relevant biosensing techniques were subsequently selected from the larger pool of physiological signatures based on their relevance in the context of measuring potential indoor environmental stress sources.

Originally, the list of biosignals retrieved from the literature review was as shown below (see Figure 4):

**Figure 4.** List of biosignals gathered from the literature review. Biosignals marked as bold are examined in this paper.

Selected biosensing techniques were then organized according to hierarchy and relevance following their prevalence in stress literature, but more importantly in building engineering stress literature, efficacy and accuracy of the results, and robustness of the system. The selected biosensing techniques were then examined in means of their physiological processes and relevance to the stress cues as investigated in this paper.

Finally, limits and potentials of these techniques described in the literature were further mentioned and discussed for all selected biosignals.

#### **3. Results**

#### *3.1. Indoor Environmental Parameters*

The factors influencing an individual's perception of the environment of a built space include a variety of aspects characterizing on one hand the physical setting itself and on the other the person's reactions to the context. The parameters in the physical space that can

trigger a (conscious or subconscious) reaction in the users essentially include (i) environmental (air quality, climate, noise, light), (ii) functional/spatial (disturbances, interruptions, distance from work, resources, plan, layout, ergonomics) and (iii) psychological factors (privacy, territoriality, aesthetics) [35,36]. This review focuses on the factors pertaining to the physical environment, and as such only the aspects under point (i) are further explored.

The physical parameters and conditions of an indoor space are mostly straightforward and objective. These can be measured according to state-of-the-art procedures and with broadly accepted units. Ranges of acceptability for human health conditions are set through national laws and standards, although in most domains research still does not agree with precision on the ranges of acceptability and healthiness [16,47]. A common way to describe the physical indoor conditions is through the assessment of the indoor environmental quality (IEQ). This system is adopted in many green building systems—the United States Green Building Council (USGBC), the Leadership in Energy and Environmental Design (LEED) and the Comprehensive Assessment System for Building Environmental Efficiency (CASBEE) [14].

Indoor environmental quality (IEQ) describes the environmental conditions inside a building in relation to the health and wellbeing of its occupants and is determined through air quality, thermal, visual, and acoustic parameters [2,132,133]. In comparison to indoor air quality (IAQ) and thermal IEQ, both visual and acoustic IEQ have received way less attention [43], as can also be seen by the number of sources reviewed in each section (see Table A7).

#### 3.1.1. Indoor Air Quality (IAQ)

Variables influencing indoor air quality (IAQ) include airborne contaminants from indoor and outdoor sources (gases and particles from equipment and furnishings, cleaning products, building materials, pollutants, etc.), ventilation rate, humidity and the type of indoor activities and the occupants' adaptive opportunities (manually intervening to adjust the conditions) [133]. Perceived IAQ is an umbrella of reported descriptors such as odor/smell, stuffy air, and dry and wet (humid) air [134].

The quality of indoor air has a direct impact on users' health and comfort [21,42,135]. Poor IAQ can directly cause severe respiratory and cardiovascular diseases, allergies, asthma, and sick building syndrome (SBS) [11,12,14,21]. As indoor air has been found to be in some cases up to five times as polluted as outdoor air [136] and new buildings tend to be increasingly tightly insulated from the external environment to achieve better energy balance [12,15,21], greater attention needs to be given to achieving good IAQ.

• Airborne Contaminants

Airborne contaminants from indoor and outdoor sources, such as gases and particles from equipment and furnishings, cleaning products, building materials, pollutants, etc., substantially affect IAQ. Common airborne contaminants include volatile organic compounds (VOCs, such as formaldehyde and benzene), microbiological volatile organic compounds (MVOCs), nitrogen oxides (NO and NO2), polycyclic aromatic hydrocarbons (PAHs) and carbon monoxide (CO) [135,137]. Carbon dioxide (CO2) is mostly not considered as a pollutant as its major source (in non-industrial indoor environments) is the human metabolism itself, but rather it is considered as a contribution to lesser air quality [53]. Finally, a great number of chemicals and materials used in everyday environments are also known to impact IAQ. A (non-exhaustive) list includes:


As a result of weak regulatory requirements for chemical safety testing, only limited toxicity data are available for many new chemicals that are being developed on a daily basis. Mechanisms of action, adverse effects, and dose–response relationships for many of these chemicals are poorly understood and no systematic screening of common chemicals for endocrine disrupting effects is currently underway, so questions remain as to the health impacts of these exposures [9]. Over the past 15 years, some chemical classes commonly used in building materials, furnishings, and consumer products have been shown to be endocrine disrupting chemicals, meaning they interfere with the action of endogenous hormones [9].

Solutions to contrast the accumulation of these pollutants include raising the ventilation rate, controlling or reducing indoor human activities such as smoking and cooking, and controlling the safety of building materials (paints, preservatives, etc.) and indoor elements (furniture, equipment, cleansers, etc.) [135,138]. Indoor greenery has been found to have positive biofiltering effects taking up toxic agents as benzene, formaldehyde, trichloroethylene [139], particulate matter (PM, fine solid particles suspended in a gas) [33] and CO2 from the air during the photosynthetic process [14,78]. Additionally, studies have suggested that buildings exhibit decreasing tendencies of VOCs in indoor environments over time, which is why old buildings tend to have lower VOC levels than new buildings [14].

Measuring of IAQ is a complex task as it has many potential components [14]. The most common methods for indoor air sampling and analyses are toxic organic (TO) methods, air pollutants in indoor air IP-methods (compendium of methods for the determination of air pollutants in indoor air) and air-phase petroleum hydrocarbon (APH) methods [140]. The emission rate of air pollution is measured in olf (from "olfactory"), where 1 olf is the emission rate of pollutants from one standard person (an average adult working in a non- industrial workplace, sedentary and in thermal comfort with a hygienic standard equivalent to 0.7 baths per day) [141]. Decipol (from "pollution") is used to represent the level of perceived air quality, where 1 decipol is the pollution caused by one standard person (one olf) ventilated by 10 l/s of unpolluted air [141]. The decipol scale ranges from 0.1 decipol (outdoors in a city) to 10 decipol (sick building). [141]. The main difficulties and limits to the measurement and mapping of VOCs are linked to their very broad diversity in physio-chemical properties [14]. This entails varying degrees of sensitivity to the diverse compounds and subjective reactions in individuals, which in turn brings difficulties in developing standard measures for sampling and analysis [14].

• Ventilation Rate

Indoor spaces are ventilated to reduce contaminants in the air and exchange the exhausted indoor air. While higher ventilation rates give better IAQ values [14], they also result in thermal exchanges between indoor and outdoor areas and therefore in higher energy consumption as the air entering needs to be treated to meet the indoor thermal comfort requirements [45]. Researchers however argue that achieving better health and productivity of occupants with higher IAQ levels results in substantial financial returns in comparison to the annual energy and maintenance costs of the building [14].

The ventilation rate is expressed as the outdoor air flow into a building per unit of time divided by the number of people in the building, and thus in liters per second (l/s) per person or per square meter.

• Humidity

Recommended levels of indoor humidity are between 40 and 60% [142]. Higher rates of humidity in indoor spaces have shown to improve sleep quality and reduce effects on the vocal cord on one hand [143], but also to create unhealthy conditions that can bring microbial growth—fungal, bacterial and mold [133,142]—and affect the rate of outgassing of formaldehyde from indoor building materials, the rate of formation of acids and salts from sulfur and nitrogen dioxide, and the rate of formation of ozone [142]. Low humidity on the other hand can favor the survival and transmission of many influenza viruses as well as aggravate the eye tear film stability and physiology, and the osmolarity of the upper airways [143]. "Dry air", which is a very common complaint in indoor spaces [134], is a sensation most likely caused by an exposure to sensory irritants such as indoor air pollutants [48,144]. Literature recommends distinguishing between elevated moisture in construction materials, elevated relative humidity (RH) resulting in condensation on surfaces, and RH in the breathing and ocular zone [143].

Relative humidity (RH) is measured as a percentage of water vapor in the room air, relative to the amount of vapor that the air could contain at a given temperature and is usually measured with a hygrometer. Absolute humidity (AH) measures the water in grams per kg of air at a defined pressure and is used in some cases for comparison and identification of associations, also considering sometimes better correlation between outdoor and indoor AH [143].

#### 3.1.2. Thermal IEQ

Thermal comfort is one of the main factors affecting occupants' perceived environmental satisfaction [2] and IEQ-productivity belief [6], influencing their health and wellbeing [60,132]. Uncomfortable thermal indoor conditions have been reported to affect cognitive performance and productivity [2,14,51] and to be in extreme cases responsible for sick building flu-like symptoms (eye, nose, and throat irritation) [2]. The range of indoor temperatures deemed acceptable also has a strong bearing on building energy requirements [51]. It is therefore understandable that the opportunity of achieving important cuts in operational CO2 emissions with the energy efficiency upgrade of the existing building stock [145] has driven much research to focus on the thermal aspect.
