1. Introduction
Since people spend more than 80% of their time in indoor environments [
1], if the indoor conditions are deficient, the health and comfort of the occupants may be affected [
2]. Building design and its characteristics are important factors of indoor conditions and, hence, the satisfaction levels of the occupants [
3]. Indoor environmental quality (IEQ) is defined as an indication that relates the health and well-being of the occupants of interior spaces with the quality of the building’s environment [
4].
The IEQ is essential in educational buildings, which are typically designed for high occupancy for long periods of the day [
5,
6]. In particular, a good indoor air quality (IAQ) is crucial to provide a healthy, safe, productive and comfortable environment [
7]. Students, teachers and other school staff are vulnerable to the impact of poor IAQ in these spaces, where concentration and intellectual work is required. Indoor air pollutants (i.e., inorganic/organic gases and biological and non-biological particles) accumulate more easily in indoor environments as a result of the building envelopes which intentionally separate occupants from the outside [
8]. Exposure to air pollutants may cause a risk of short- and long-term health problems, such as several respiratory diseases [
9,
10], cardiovascular disease [
11], irritated eyes or nose, blocked nose, headaches and so forth [
12]. In addition, poor IAQ may affect the comfort, productivity and academic achievement of students [
6,
13,
14]. Therefore, IAQ is of particular concern in teaching‒learning spaces.
These circumstances determine that one of the most demanding challenges facing educational building administrators is IAQ managing [
15]. An adequate ventilation rate (VR) is one of the key elements to avoid compromising the IAQ since providing outdoor air ventilation dilutes internally generated contaminants to levels that do not cause health and comfort problems [
16]. The analysis of the VR based on measured studies and the adequately characterised ventilation design of buildings are critical for assessing and interpreting IAQ [
17,
18]. Selecting an appropriate ventilation strategy is essential for meeting the requirements for good IAQ. International guidelines, standards and building codes state a minimum VR in buildings [
19,
20,
21,
22]. However, it should be noted that previous research suggests that in order to substantially decrease illness absence and therefore produce economic benefits, one of the measures that can be taken is to increase classroom VRs above the State standard [
23].
This fact has been highlighted by the COVID-19 pandemic. According to the World Health Organization, as of 7 July 2021, there had been 3,997,640 deaths and 184,572,371 confirmed cases of COVID-19 reported globally [
24]. Transmission of SARS-CoV-2 occurs when uninfected people are exposed to infectious respiratory fluids after contact with infected people [
25]. Factors contributing to increased transmission include: loud speech volume; intense physical activity; lack of well-fitting face masks; large numbers of people in the same space; decreased interpersonal distance; increased emission and exposure time and poor indoor VR [
26]. Moreover, recent research has shown that transmission can be aggravated in confined and poorly ventilated spaces. Indeed, Nishiura et al. [
27] state that COVID-19 transmission can be up to 18.7 times higher in confined spaces than in open air spaces. Park et al. [
28] suggested that cross-ventilation is more efficient compared to single-sided ventilation, and recommend cross-ventilation to minimise the possibility of infection in high-density public buildings. According to Dai and Zhao [
29], for a classroom with a volume of 348 m
3 and for an exposure period of 2 h, to keep the probability of infection below 1%, a VR of two Air Changes per Hour (
ACH) with masks and seven
ACH without masks is necessary.
Since students and teachers spend long periods each day in classrooms, these indoor spaces are risk environments for the airborne transmission of SARS-CoV-2 [
30]. Consequently, measures adopted by governments to minimise the possibility of contagion included the closure of educational buildings. As a result, nearly half of the world’s students are still affected by this measure and more than 100 million additional children will fall below the minimum level of reading proficiency [
31]. The United Nations Educational, Scientific and Cultural Organization (UNESCO) warns that it is crucial to prioritise education recovery in order to avoid a generational catastrophe [
31]. Adopting effective mitigation strategies to control the risk of airborne infection and adapting educational-learning spaces are essential processes to mitigate the impact of educational building closures. The reopening of educational buildings has had many socio-economic implications in all countries, and therefore countries are taking actions to ensure that educational buildings are safe spaces. In this regard, the Spanish Government’s prevention guidelines require the use of well-fitted facemasks (a surgical mask is a minimum), reducing the volume of the voice in conversation, increased interpersonal distance and reduced contact time (e.g., reducing the occupation of indoor spaces) and improved ventilation in indoor spaces. Ventilation strategies are a key aspect of indoor spaces management in this context. In the case of natural ventilation, cross-ventilation (opening doors and/or windows on opposite sides) is recommended [
26]. For mechanical ventilation, attention should be paid to the configuration of the system, to reduce the recirculation of air and increase/maximise outside air. The VR is measured by ACH. The recommended VR in indoor spaces for good air quality is 12.5 litres/second per person (L/s/p), which corresponds to approximately 5–6 ACH.
However, while these ventilation strategies ensure an optimal concentration of CO
2 and other pollutants, they also have an impact on other important indoor variables in indoor environments. One of the most important in teaching‒learning spaces is the indoor acoustic environment, which is influenced by the natural and/or mechanical ventilation strategy selected [
32]. In recent years, perceived acoustic quality in indoor environments has gained momentum and recent research has focused on indoor soundscapes [
33,
34] Acoustic design and strategies should include noise control and perceptual approach of the users in order to enhance people’s health and well-being [
35,
36]. In this sense, Tang [
37] analysed available façade noise control strategies for introducing devices while improving natural ventilation in buildings. The findings of his study show that, in congested cities, protrusive devices such as balconies, lintels and fins are not effective noise screening devices for high-rise buildings (even with sound absorbers and/or reflectors). Active control installation and resonance-based devices often result in bulky systems, affecting the façade design and the effectiveness of natural ventilation strategies. Systems such as plenum windows and double-wall plenum structures are often useful as natural ventilation and noise control devices. In addition, research is being conducted on the development of new window devices. Fusaro et. al. [
38,
39] proposed a new metacage window which allows natural ventilation and noise reduction based on the principle of Snell’s Law. The used of this novel prototype showed an overall mean sound reduction of 15 dB within a bandwidth of 380 to 5000 Hz.
In this context, the management of natural ventilation strategies and their impact in the indoor acoustic environment is essential in the teaching‒learning spaces. Poor acoustic environments in classrooms affect learning achievements [
40,
41] as well as the academic, psychosocial and psychoeducational performance of students [
42]. Moreover, these may cause voice problems [
43] and physical stress in teachers [
44], and have significant effects on word identification and intelligibility [
45]. External noise sources to educational buildings as well as sources within the building (e.g., in facilities rooms, contiguous spaces, etc.) influence the background noise inside the teaching‒learning spaces. In order to achieve an adequate acoustic comfort and speech intelligibility to ensure the quality of educational processes the background noise level should not exceed the sound level of 35 dBA [
46,
47]. Therefore, acoustic comfort is critical in determining the quality of educational processes. This fact makes it necessary to evaluate the impact of the ventilation strategies on IEQ parameters such as IAQ and acoustic performance. This is the main general purpose of this research.
In this context, and given the 6 ACH values recommended in current Spanish public policies to prevent the transmission of COVID-19, the aim of this study was to characterise their impact on the variables conditioning IAQ and the indoor acoustic environment. The study assesses the need to define health protocols for ventilation in educational buildings that, in addition to identifying natural ventilation strategies with a VR value as close as possible to the required ACH value, take into account the background noise level. This will therefore ensure the quality of teaching and learning processes while maintaining the required ventilation protocols.
4. Discussion
The classrooms selected in this study are representative of the classrooms’ typology in Building 1 (Building Engineering School) and Building 2 (Civil Engineering School) of the University of Granada during return to teaching activity. For this purpose, field measurements were carried out to test three different configurations in the four selected classrooms.
The results obtained from the experimental tests in Building 1 showed that, in classroom B1-A1, the configuration that provides the lowest average ACH value is configuration C-3 (4.6) and the highest average value is configuration C-1 (8.3). In the case of the classroom B1-A2, the configurations providing the lowest and highest average value of ACH are configuration C-3 (3.7) and C-1 (6.1) respectively. With regard to the results obtained from the experimental analysis in Building 2, in the case of classroom B2-A1, configuration C-3 provides the lowest mean ACH value (8.4) and configuration C-1 provides the highest mean ACH value (24.9). In the case of classroom B2-A2, the configuration providing the lowest average ACH value is configuration C-3 (6.1) and the highest average ACH value is configuration C-1 (15.5).
As can be seen, the VR depends on the local and particular conditions of each indoor space. In this context, the configuration chosen among the three analysed in each classroom was the one that meets the minimum ventilation requirements. The configurations selected for classrooms B1-A1, B1-A2, B2-A1 and B2-A2 were configurations C-1 (all windows opened and main door opened), C-1 (all windows opened, main door opened and the corridor windows opened), C-3 (only windows at the end opened and main door opened) and C-2 (all windows opened and the main door opened) respectively. This decision is based on ensuring that the ACH value is sufficient to guarantee that the space is safe, although there may be variability in the ACH value due to possible variations in environmental conditions.
Once the natural ventilation configuration was selected for each classroom, an acoustic study was carried out to compare the normal classroom scenario (windows and door closed) with the chosen configuration of natural ventilation. As can be seen from the results obtained, since the background noise level should not exceed 35 dBA for good speech intelligibility, none of the classrooms met this acoustic quality recommendation. With regard to the comparison between the scenario of closed doors and windows and the natural ventilation configuration selected, it was identified that the natural ventilation configuration causes an increase of between 6.4 dBA and 12.6 dBA in the background noise level of the classrooms analysed. The background noise is an important factor that affects the acoustic clarity and quality of teaching and learning process [
56].
Background noise is closely related to the signal-to-noise ratio (SNR). In this sense, a high level of background noise can cause a low or negative SNR. Therefore, a poor SNR causes, on the one hand, difficulties for students having to understand the message. On the other hand, it also causes a higher vocal effort among teachers, as the speaker’s speech level has to be higher than the background noise level.
In fact, background noise becomes a problem that has a major impact on the current situation. Since the classrooms used for the return to campus are larger, and to ensure physical distance between students the distribution of students occupies all rows of seats, many students are in positions far away from the teacher. As a result, the signal‒noise ratio is very low in the rear positions, causing significant effects on reducing word identification and intelligibility.
The location and orientation of the classroom also influences the impact of the natural ventilation configuration on classroom background noise. This is evident in the results obtained for classroom B1-A1, which is oriented towards a dense traffic area and the background noise level was 54.1 dBA. Therefore, more factors than room size and ventilation strategy should be taken into account when choosing the classroom. The location and orientation of the classroom should be considered in order to reduce the impact of background noise on the teaching‒learning process. Consequently, the practical implications of the findings show that ventilation strategies management in educational buildings should consider the following design and operation guidelines:
The classroom selection must take into account both the health recommendations and the impact of background noise. Priority should be given to selecting those indoor spaces that: 1) meet the health requirements (minimum distances, VR, etc.) and 2) (due to their location and orientation) have a background noise level that does not interfere with the teaching-learning activities.
In those cases where it is not possible to meet the criterion stated in the previous point, an adaptation intervention must be carried out (i.e., installation of passive, active, automation-based or hybrid noise control devices). Noise control solutions for natural ventilation openings must ensure the required VR while also ensuring the background noise does not interfere with the performance of students and teachers.
The limitations presented in the study stem from the effect of indoor and outdoor environmental conditions (the local and particular conditions of each indoor spaces as well as the wind speed and outdoor temperatures). Additionally, this study follows the protocols stated by the Spanish Government and University of Granada prevention guidelines. One of this protocols is the IAQ management of both buildings is to ventilate (for at least 1 h before and after each class) by opening all windows. This procedure achieves indoor temperature and relative humidity levels similar to those outside, so the effect of these factors should be taken into account if different conditions would apply.
5. Conclusions
The aim of this study was to analyse the natural ventilation strategies through the configuration of window and door openings, in accordance with the recommendations established in the COVID Action Plan of the University of Granada, which complies with the recommendations while maintaining the maximum degree of comfort for the user. To this end, the impact of these measures on the acoustic environment of the classroom was analysed, so that both students and teaching staff maintain safe levels of protection against the transmission of SARSCOV-2 without affecting their teaching‒learning activities.
The results obtained show that a correct choice of configuration can satisfy the VR needs while ensuring that the indoor space is safe for the occupants. The measurements were carried out in four different classrooms with an occupancy per area ranging from 2.20 m2/student to 4.77 m2/student. These spaces were selected according to the COVID-19 contingency plan set up at the beginning of the 2020/2021 academic year in each university centre. The natural ventilation configuration that met the required ACH was chosen to assess the impact on background noise inside the classroom. The main results obtained were:
Natural cross-ventilation is an effective strategy to achieve the ACH levels required to ensure that the indoor spaces meet the guideline recommendations for a safe return to campus.
There are differences in the specific natural ventilation strategy depending on the configuration of classrooms and building orientation. Thus, for the classrooms in building B1 the configuration of all windows opened and main door opened should be selected no matter the type of possible ventilation (natural ventilation through windows or cross-ventilation through corridors). On the other hand, in B2 the specific configuration depends on the classroom type, i.e., all windows opened and main door opened in the case of south-orientated classroom, or only windows at the end opened and main door opened in the case of the case of north-orientated classroom achieve better results due to the different orientation of the building. This fact highlights the needs of performing specific studies to select the best strategy to implement natural cross-ventilation.
The average VR value provided by the selected configuration for each classroom was 8.3 ACH, 6.1 ACH, 8.4 ACH and 8.8 ACH for classrooms B1-A1, B1-A2, B2-A1 and B2-A2, respectively. Therefore, the average ACH value is above 6 ACH in all the selected natural ventilation configurations.
The background noise level is strongly affected by the selected natural ventilation configuration. The background noise levels with the selected natural ventilation configuration were between 43.2 and 54.1 dBA. As can be seen, all classrooms exceed the recommended 35 dBA background noise level limit for background noise in teaching spaces. Consequently, the teaching activity management has to take into account not only the ACH, but also its impact on the indoor environmental conditions such as the acoustic environment. Since a high value of background noise level can interfere with the teaching and learning process and even interfere with the performance of students and teachers, educational building administrators need to consider this issue. In those cases where in order to achieve a natural ventilation strategy that provides the required VR, the background noise level exceeds 35 dbA, building managers should make intervening adaptations (i.e., installation of passive, active, automation-based or hybrid noise control devices).
Since this research proves that the best strategies to achieve a VR value that complies with the standard imply a significant impact in other indoor environmental variables such as indoor noise levels, some actions to improve the indoor acoustic behaviour of classrooms are recommended. For example, the need of electroacoustic support to increase speech intelligibility, improving the acoustic conditioning of classrooms, increasing noise insulation with other classrooms and other common areas, and reinforcing the compliance of outdoor noise levels achieving the acoustic quality criteria prescribed for sensitive acoustic areas such as the educational ones. Therefore, the management, organization and planning for indoor spaces of educational buildings must not only ensure occupants’ safety, but also not influence the performance of teaching activities. Action plans are required that allow buildings’ administrators to achieve adequate natural ventilation strategies and implement effective noise reduction measures in indoor spaces.
Finally, future studies should focus on the environmental conditions of natural ventilation with occupancy in the classrooms, in order to evaluate not only the objective variables of the IEQ factors, but also the subjective variables associated with the perception and comfort of occupants with regard to the window and door opening configurations established.