Effects of the COVID-19 pandemic on indoor air quality and thermal comfort of primary schools in winter in a Mediterranean climate

sustainability Article Effects of the COVID-19 Pandemic on Indoor Air Quality and Thermal Comfort of Primary Schools in Winter in a Mediterranean Climate Alicia Alonso 1,* , Jesús Llanos 2, Rocío Escandón1and Juan J. Sendra 1   Citation: Alonso, A.; Llanos, J.; Escandón, R.; Sendra, J.J. Effects of the COVID-19 Pandemic on Indoor Air Quality and Thermal Comfort of Primary Schools in Winter in a Mediterranean Climate. Sustainability 2021,13, 2699. https://doi.org/ 10.3390/su13052699 Academic Editor: Paola Villoria-Sáez Received: 5 February 2021 Accepted: 26 February 2021 Published: 3 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. 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/). 1Instituto de Arquitectura y Ciencias de la Construcción, Escuela Técnica Superior de Arquitectura, Universidad de Sevilla, 41012 Sevilla, Spain; [email protected] (R.E.); [email protected] (J.J.S.) 2 Escuela Técnica Superior de Arquitectura, Universidad de Sevilla, 41012 Sevilla, Spain; [email protected] *Correspondence: [email protected] Abstract: Research studies have shown the potential effects of indoor environmental quality (IEQ) on pupils’ health and academic performance. The COVID-19 pandemic has prompted renewed interest in the assessment of deficient indoor air quality (IAQ) conditions in schools and has become a priority over achieving adequate comfort conditions. Scientific studies confirm aerosols as one of the transmission routes of SARS-CoV-2 so that the possibility of airborne transmission increases in indoor environments with high occupancy, such as classrooms. As a result, international protocols and guidelines have established a requirement for educational buildings to over-ventilate with a fresh outdoor air supply. The main object of this work is to analyse the effects of the COVID-19 pandemic on thermal comfort and indoor air quality, in winter, in two classrooms of southern Spain. Thus, onsite measurements of environmental variables were conducted before and during the pandemic. Both classrooms have mechanical ventilation systems as they are within a recently built primary school (2018). Results shows a decrease of 300 ppm in CO 2 weekly average values during the pandemic, when hybrid ventilation is used, and a decrease of 400 ppm when schools are naturally ventilated during all teaching hours. However, the analysis of standards shows that over 60% of hours are thermal discomfort conditions. Keywords: COVID-19; IAQ; children’s school; thermal comfort; IEQ; Mediterranean climate; manual airing 1. Introduction In recent years, there have been many studies on indoor environmental quality (IEQ) in schools in different countries and climatic areas. The scientific literature has widely analysed the vulnerability of students to indoor environmental conditions in high-occupancy spaces and its potential effect on pupils’ health and wellbeing [ 1 – 3 ], as well as on the learning process and academic performance [ 4 – 6 ]. A guideline book was also published to describe the optimal design and operation of schools with respect to low energy cost and performance of students, focusing on healthy indoor environment among other aspects [ 7 ]. Most of these studies, several of which took place in the Mediterranean climate, reported frequent deficiencies in indoor environmental conditions [ 8 , 9 ], mainly the indoor air quality (IAQ), resulting from inadequate or misused ventilation systems [ 10 – 12 ]. The study by Campano et al. [ 8 ] concluded that heating did not lead to widespread complaints as the occupants’ mean thermal sensation (TSV) in classrooms in a Mediterranean climate is that of thermal neutrality during winter, and slight warmth during mid-seasons. In this regard, the school environment must be homogeneously clean, safe, and comfortable in order to reduce exposure to chemical and biological agents, prevent diseases, facilitate cognitive development, and avoid unequal educational outcomes [ 13 ]. D’Ambrossio Alfano et al. [ 14 ] developed other study that should be mentioned, in which there are Sustainability 2021,13, 2699. https://doi.org/10.3390/su13052699 https://www.mdpi.com/journal/sustainability Sustainability 2021,13, 2699 2 of 17 also wider thermal comfort analyses over a large sample of students (4000) carried out in Mediterranean Countries, such as Italy. The study concluded that the Fanger’s basic approach for the assessment of the thermal comfort is also effective in naturally ventilated environments if a right expectancy factor is known. In spite of this, neither the public nor private administration has focused special attention on the IAQ of schools, and more surprisingly, it has also been neglected by the educational community and parents’ associations. CO 2 is a good bio-effluents indicator, and it could be correlated with the ventilation of a room, therefore, its concentration rate is used as the main indicator of IAQ [ 15 ] and to determine the ventilation rate in occupied spaces [ 16 ] such as schools. However, if the levels of pollution are higher outside than indoors, CO 2 does not indicate indoor pollutants with health risk [ 17 ]. Except for a few studies, the monitoring of CO 2 levels in classrooms and the relationship of these levels to the use of ventilation systems have barely been studied [ 18 ]. Furthermore, it has also not been verified whether the average level of 1000 ppm, recommended by some international documents and standards [19–21], has been exceeded. The COVID-19 pandemic, declared by the World Health Organisation (WHO) on 11 March 2020, has prompted renewed interest in the assessment of deficient IEQ conditions in school classrooms, mainly indoor air quality and thermal comfort, making it necessary to completely modify how educational spaces are ventilated. Based on the findings of different studies, the transmissibility of COVID-19 can be influenced by airflow, as well as by environmental conditions. Recently, research has pinpointed aerosols as one of the transmission routes of SARS-CoV-2 [ 22 ] so that airborne transmission in indoor environments is highly likely [23]. In March 2020, school closures were implemented worldwide as a measure for preventing the spread of COVID-19. The repercussions of this preventive measure on educational and social development, wellbeing, and mental health were negative [ 24 , 25 ]. Although it has been concluded that the reopening of schools has not led to significant outbreaks or increased risk [ 26 , 27 ], some studies established that a return to the classroom must be accompanied by large-scale Test, Trace, and Isolate (TTI) systems [ 28 ] in order to prevent new waves. Recent research and international guidelines published for schools’ reopening [ 25 , 29 ] have determined the need to implement effective measures to minimise the risk of transmission, including an interpersonal distance of 2 m, face masks, frequent hygiene, temperature checks, and improved ventilation [ 23 , 24 , 30 ]. According to results, natural, hybrid, and mechanical ventilation systems can be used effectively and safely in schools, removing contaminated indoor air and supplying fresh outdoor air [ 26 ]. This preventive measure, along with the addition of filters, is a primary solution for the control of infectious diseases [ 31 , 32 ] and reduction of potential transmission [ 33 ]. However, not all ventilation strategies are acceptable and those which rely on centralised air distribution and/or recirculation create optimum conditions for the rapid spread of disease [16]. According to several authors, the recommended rates established in standards [ 34 , 35 ], 4–6 air change hours (ACH) for classrooms [ 29 ] and a relative humidity (RH) of 40–60%, may be low in pandemic conditions [32]. The problem of poor environmental conditions is especially serious in classrooms of schools in countries with a Mediterranean climate, including Spain, as most schools rely on the manual opening of windows as the only ventilation system to try to achieve good IAQ [ 10 ], which is expected to worsen in the climate change scenarios forecast [ 36 ]. Increasing outside air fractions in a Mediterranean climate entails a rise of total air change rates, but it may also lead to higher energy consumption. This is probably one of the main reasons why approximately 8000 educational establishments in the school building stock of the Mediterranean area do not have mechanical ventilation systems [ 37 ], even though Spanish regulations specify the need for this type of ventilation so that IAQ is classed as “IDA 2”, category of IAQ (CO 2 level of 500 ppm additional to the outdoor air concentration), requiring a ventilation flow of 0.0125 m3/s per occupant [38]. Sustainability 2021,13, 2699 3 of 17 In this regard, in adverse weather conditions, the widespread use of manual airing does not usually guarantee a decreased health risk of spreading airborne diseases and outdoor air pollution, nor does it ensure the conditions of thermal comfort and energy consumption. Research carried out by Stabile et al. in Italian schools, which rely predominantly on manual airing, provides information on the incompatibility between hygrothermal comfort and the IAQ of classrooms in winter conditions [ 39 ]. The effect of some ventilation strategies on indoor air quality and heating consumption in classrooms is also analysed, with the presence of energy-deficient thermal envelopes also affecting the situation [ 40 ]. Thus, in order to maintain hygrothermal comfort [ 41 ], a balance should be struck by ensuring manual airing during breaks, thus reducing CO 2 levels without altering the thermal comfort of pupils [42]. Before the COVID-19 pandemic, achieving adequate comfort conditions was considered a priority over ensuring an adequate IAQ. However, currently, for winter 2020–2021, protocols and guidelines for classroom ventilation have been established in Spain [ 43 , 44 ], even with a clear decrease in the recommended limit values for CO 2 levels (Figure 1). Although existing guidelines do not follow a systematic approach when defining measures, they prioritise guaranteeing IAQ in order to ensure the greatest possible prevention of transmission, reducing anything that might significantly affect not only hygrothermal comfort conditions but also pupils’ health. Sustainability 2021, 13, x FOR PEER REVIEW 3 of 17 “IDA 2”, category of IAQ (CO₂ level of 500 ppm additional to the outdoor air concentration), requiring a ventilation flow of 0.0125 m3/s per occupant [38]. In this regard, in adverse weather conditions, the widespread use of manual airing does not usually guarantee a decreased health risk of spreading airborne diseases and outdoor air pollution, nor does it ensure the conditions of thermal comfort and energy consumption. Research carried out by Stabile et al. in Italian schools, which rely predominantly on manual airing, provides information on the incompatibility between hygrothermal comfort and the IAQ of classrooms in winter conditions [39]. The effect of some ventilation strategies on indoor air quality and heating consumption in classrooms is also analysed, with the presence of energy-deficient thermal envelopes also affecting the situation [40]. Thus, in order to maintain hygrothermal comfort [41], a balance should be struck by ensuring manual airing during breaks, thus reducing CO₂ levels without altering the thermal comfort of pupils [42]. Before the COVID-19 pandemic, achieving adequate comfort conditions was considered a priority over ensuring an adequate IAQ. However, currently, for winter 2020–2021, protocols and guidelines for classroom ventilation have been established in Spain [43,44], even with a clear decrease in the recommended limit values for CO₂ levels (Figure 1). Although existing guidelines do not follow a systematic approach when defining measures, they prioritise guaranteeing IAQ in order to ensure the greatest possible prevention of transmission, reducing anything that might significantly affect not only hygrothermal comfort conditions but also pupils’ health. Figure 1. Maximum recommended CO₂ concentration in a classroom based on occupancy (considering 450 ppm of CO₂ in outdoor air). Blue line: maximum recommended values. Red line: limit values that should not be exceeded at any time [43]. Conditions of vulnerability are especially pronounced in pre-schools and primary schools [45,46], due to the age of the potential students: 4 to 6 years of age in the former and 6 to 12 in the latter. Avoiding crowded, poorly ventilated classrooms is a requirement for the safe and healthy return of most pupils to school [29,47]. The implementation of a system of hygrothermal conditions and IAQ monitoring (air temperature (ta), RH, CO₂), together with other interventions, is conditional on the success of not exceeding acceptable limits [24]. Figure 1. Maximum recommended CO 2 concentration in a classroom based on occupancy (considering 450 ppm of CO 2 in outdoor air). Blue line: maximum recommended values. Red line: limit values that should not be exceeded at any time [43]. Conditions of vulnerability are especially pronounced in pre-schools and primary schools [ 45 , 46 ], due to the age of the potential students: 4 to 6 years of age in the former and 6 to 12 in the latter. Avoiding crowded, poorly ventilated classrooms is a requirement for the safe and healthy return of most pupils to school [ 29 , 47 ]. The implementation of a system of hygrothermal conditions and IAQ monitoring (air temperature (ta), RH, CO 2 ), together with other interventions, is conditional on the success of not exceeding acceptable limits [24]. Going to schools is an essential activity for children and young students, so it is important to analyse pupils’ thermal comfort and IAQ conditions of classrooms. It should also be noted the social significance that IEQ, manifested by the media, has had in the classrooms of schools during this year of pandemic. The main objective of this work is to analyse the Sustainability 2021,13, 2699 4 of 17 monitored indoor environmental conditions in winter for two classrooms in southern Spain. Having had the opportunity to monitor the environmental variables of a primary school just before the pandemic, the results presented for this study are for January 2020, before the pandemic, and for December–January 2021, during the pandemic. These classrooms incorporate a mechanical ventilation system, as they are in a recently built pre-school (2018). However, under normal conditions of use (prior to the pandemic), it used both natural and mechanical ventilation, depending on the external environmental conditions. 2. Materials and Methods The research methodology carried out starts with the implementation of a system of thermal comfort and IAQ monitoring, which measured environmental parameters during the normal operation of teaching activities for a period of 13 weeks in winter before and during the COVID-19 pandemic. The selected case study is made up of two representative classrooms of a pre-school located in southern Spain (Mediterranean climate). In order to obtain results and analyse the hygrothermal and air quality conditions and effects of the pandemic on the selected case study, the methodological process was set out as follows: •Selection and characterisation of case study, •Development of monitoring campaign, • Data collection: use of heating, ventilation, and air conditioning (HVAC), ventilation protocols, thermal comfort, and IAQ parameters. 2.1. Characterisation of the Case Study The pre-school selected in this study is located in Seville, which is classified as B4 following the Spanish energy performance zoning and is one of the most representative climatic areas in the south of Spain. Different areas in the region of Andalusia, including Seville, are covered according to the Köppen climate classification [ 48 ]. Zone B4 is characterised by hot-summer Mediterranean climate (Köppen CSa), with temperatures above 22 ◦ C, and cold to temperate areas in winter, with minimum temperatures usually remaining above 0 ◦C. The recent construction date (2018) and the strict regulations in place justify the suitable thermal characteristics of the building envelope (Table 1), whose thermal transmittance (U) values comply with the limit established [ 49 ]. In addition, this school has mechanical ventilation systems, mandatory in Spain since 2007, but lacking in most of the school building stock of the Mediterranean [ 35 ], due to its high cost, energy consumption, and demanding maintenance regime. The school is located in the East of Seville, next to a large green area, which contributes to the low level of outdoor air pollution. The two classrooms selected are located on the second floor of the building and they are geometrically identical, but with opposing orientation: one is south facing (A5), while the other faces north (A6). In addition, although both have a large floor-to-ceiling window onto a courtyard, the position of the windows on the facade differs. Although both classrooms have a mechanical ventilation system, regarding manual airing through windows, only A5 can provide cross-ventilation while A6 only provides ventilation from one side. Figure 2shows images of the building and one of the classrooms analysed in this study. Table 2collects occupancy patterns, air rate, thermal loads, and other HVAC information of interest. Sustainability 2021,13, 2699 5 of 17 Table 1. Classroom envelope composition and thermal transmittance value (U). Element Composition Thermal Transmittance U Limits Thickness U (W/(m2K)) (W/(m2K)) [49](cm) Facade Concrete prefabricated panel (16 cm), air chamber of variable thickness, self-supporting framework with reinforced plasterboard panels (15 + 15 mm) and mineral wool insulation (40 mm). 0.57 1 27 Windows Aluminium frame (Class 2 air permeability, Class 6A watertightness, and wind resistance Class C2). (A5, A6) Floor-to-ceiling windows of laminated safety glass (6 + 6 mm). (A5) Sliding windows—insulated glass units (4 + 12 + 6 mm). 4.06 4.2 - Roof Reinforced concrete slab (35–40 cm high) and lightweight concrete blocks. False ceiling: air chamber, mineral wool insulation (20 mm), and plasterboard panel (13 mm). Non-transitable roof covering with precast concrete flooring 0.37 0.65 100–120 Sustainability 2021, 13, x FOR PEER REVIEW 5 of 17 Table 1. Classroom envelope composition and thermal transmittance value (U). Element Composition Thermal Transmittance U Limits Thickness U (W/(m2K)) (W/(m2K)) [49] (cm) Facade Concrete prefabricated panel (16 cm), air chamber of variable thickness, self-supporting framework with reinforced plasterboard panels (15 + 15 mm) and mineral wool insulation (40 mm). 0.57 1 27 Windows Aluminium frame (Class 2 air permeability, Class 6A watertightness, and wind resistance Class C2). (A5, A6) Floor-to-ceiling windows of laminated safety glass (6 + 6 mm). (A5) Sliding windows— insulated glass units (4 + 12 + 6 mm). 4.06 4.2 - Roof Reinforced concrete slab (35–40 cm high) and lightweight concrete blocks. False ceiling: air chamber, mineral wool insulation (20 mm), and plasterboard panel (13 mm). Non-transitable roof covering with precast concrete flooring 0.37 0.65 100–120 The two classrooms selected are located on the second floor of the building and they are geometrically identical, but with opposing orientation: one is south facing (A5), while the other faces north (A6). In addition, although both have a large floor-to-ceiling window onto a courtyard, the position of the windows on the facade differs. Although both classrooms have a mechanical ventilation system, regarding manual airing through windows, only A5 can provide cross-ventilation while A6 only provides ventilation from one side. Figure 2 shows images of the building and one of the classrooms analysed in this study. Table 2 collects occupancy patterns, air rate, thermal loads, and other HVAC information of interest. Figure 2. Images of the building and one of the classrooms. Table 2. Classroom characteristics. Parameter South Class (A5) North Class (A6) Area 105.30 m 2 100.15 m 2 % of gaps in the envelope 30.6% 22.7% Envelope gap characteristics Class 2 air permeability. U = 4.06 W/m 2 ·K Occupation Profile 25 5-year-old children + 1 teacher Occupation ratio 4.05 m 2 /person 3.85 m 2 /person Hours of occupation 9:00–14:00 h Required IAQ category: “IDA 2” HVAC * systems Mechanical ventilation: Heat recovery (43–50% of efficiency) Discharge flow: 0.380 m 3 /s (per classroom) Figure 2. Images of the building and one of the classrooms. Table 2. Classroom characteristics. Parameter South Class (A5) North Class (A6) Area 105.30 m2100.15 m2 % of gaps in the envelope 30.6% 22.7% Envelope gap characteristics Class 2 air permeability. U = 4.06 W/m2·K Occupation Profile 25 5-year-old children + 1 teacher Occupation ratio 4.05 m2/person 3.85 m2/person Hours of occupation 9:00–14:00 h Required IAQ category: “IDA 2” HVAC * systems Mechanical ventilation: Heat recovery (43–50% of efficiency) Discharge flow: 0.380 m3/s (per classroom) 0.0152 m3/s·person Variable Refrigerant Volume system (VRV) Outdoor Unit: Heat Pump Indoor Unit: Duct Fancoil * Heating, Ventilation and Air Conditioning. Sustainability 2021,13, 2699 6 of 17 Figure 3shows the floor distribution, the position of monitoring devices, and a distribution scheme of HVAC facilities in the two classrooms. The ventilation system is predominantly solved by a mixed air HVAC system (Figure 3), with VRV (Variable Refrigerant Volume) direct expansion equipment for hot/cold thermal treatment. Therefore, a heat pump is used for thermal production and sensible heat recovery equipment for the mechanical ventilation system, scheduled to operate during occupied hours. The IAQ established in this building is categorised as “IDA 2” [ 38 ] with a CO 2 level of 500 ppm in addition to the outdoor air concentration, requiring a 0.0125 m 3 /s ventilation flow per occupant. Sustainability 2021, 13, x FOR PEER REVIEW 6 of 17 0.0152 m3/s·person Variable Refrigerant Volume system (VRV) Outdoor Unit: Heat Pump Indoor Unit: Duct Fancoil * Heating, Ventilation and Air Conditioning. Figure 3 shows the floor distribution, the position of monitoring devices, and a distribution scheme of HVAC facilities in the two classrooms. The ventilation system is predominantly solved by a mixed air HVAC system (Figure 3), with VRV (Variable Refrigerant Volume) direct expansion equipment for hot/cold thermal treatment. Therefore, a heat pump is used for thermal production and sensible heat recovery equipment for the mechanical ventilation system, scheduled to operate during occupied hours. The IAQ established in this building is categorised as “IDA 2” [38] with a CO₂ level of 500 ppm in addition to the outdoor air concentration, requiring a 0.0125 m3/s ventilation flow per occupant. Figure 3. Plans of case studies with a distribution scheme of heating, ventilation, and air conditioning (HVAC) facilities in the two classrooms (red points show the position of monitoring devices): (a) roof plan, (b) floor plan. 2.2. Monitoring The database is established from onsite measurements of the environmental conditions of air temperature, relative humidity, and level of CO₂ concentration. The monitoring device used was the Wöhler CDL 210 logger and data collection was configured to take measurements at 30 min intervals. Table 3 summarises the characteristics of the equipment and sensors used in the measurement. Door Door Windows Figure 3. Plans of case studies with a distribution scheme of heating, ventilation, and air conditioning (HVAC) facilities in the two classrooms (red points show the position of monitoring devices): (a) roof plan, (b) floor plan. 2.2. Monitoring The database is established from onsite measurements of the environmental conditions of air temperature, relative humidity, and level of CO 2 concentration. The monitoring device used was the Wöhler CDL 210 logger and data collection was configured to take measurements at 30 min intervals. Table 3summarises the characteristics of the equipment and sensors used in the measurement. As shown in Figure 3, monitoring devices were placed at a height of approximately 1.5 m, along the interior perimeter of the classroom away from direct solar radiation and air currents to avoid distortions in the data collection without hampering teaching activity. It was located in a representative point where users can have control and perception of the temperature, taking into account the importance of making these criteria compatible with the conditions of use of a space occupied by children. Sustainability 2021,13, 2699 7 of 17 Table 3. Characteristics of the measuring equipment. CDL 210 Wöhler Data Logger Parameter Units Limit Range Accuracy Measuring Interval CO2concentration Ppm 0–6000 50 (±5%) 15 min Air temperature (ta) ◦C−10–60 ±0.6 Relative humidity (RH) % 5–95 ±3 Data for thermal effects were recorded over approximately 13 weeks in winter. An analysis of results will be carried out for two periods, before and during the COVID-19 pandemic, during which schools modified the ventilation patterns following the established protocols and guidelines. The periods correspond to the following dates: • Period 1: From 23 January to 13 March 2020, the last day of face-to-face teaching activity before the lockdown imposed due to the COVID-19 pandemic. In total, this 7-week period is the equivalent of 35 school days. • Period 2: From 30 November 2020 to 22 January 2021. In total, this 6-week period is the equivalent of 25 school days, including the Christmas holidays. 2.3. Thermal Comfort Analysis To evaluate the effect of over-ventilation in thermal conditions, the comparative analysis is complemented with objective assessment of thermal comfort level by establishing parameters according to the thermal sensation experienced by humans. In accordance with recommendations of different methods and standards, thermal comfort models, both static (RITE and ISO 7730) [ 37 , 50 ] and adaptive (ASHRAE 55 and EN 16798) were analysed [ 10 , 51 , 52 ]. This task was carried out using different data, such as environmental parameter measurements, metabolic rate, and/or insulation clothing level. As stated by D’Ambrossio Alfano et al. [ 53 ], the predicted mean vote (PMV) model requires the knowledge of four thermohygrometic parameters (air temperature, radiant temperature, air velocity, and relative humidity). Limitations of this study promoted by the haste and urgency of reproducing the situation of the pandemic led to determine an approximation of the model through comfort temperature bands. According to the current Spanish regulations [ 38 ], thermal comfort is defined as an operative temperature range of 21–23 ◦ C in winter. When applying the method detailed in ISO 7730 [ 50 ], the metabolic rate value considered was 1.40, met for 5-year-old children, with a clothing insulation value of 1 clo for the winter period, based on the clothing worn by the occupants, and quantified according to Section A.4 of the ISO 7730 document. The thermal environment category selected was “B”, which implies a Predicted Percentage of Dissatisfied (PPD) below 10%. The adaptive model defined in standard EN 16,798 [ 51 ] is applicable only to buildings where occupants perform low metabolic rate activities and can easily access operable windows and freely adapt clothing according to thermal conditions. In this study, a normal level of expectation (Cat. II level) is used, corresponding to a PPD < 10% and an acceptability range around the optimum comfort temperature of ±3◦C. Regarding ASHRAE Standard 55 [ 20 ], this method is also applied in buildings used for low metabolic rate activity. For this analysis, the range considered is 90% of occupant acceptability (PPD < 10%) (Cat. I level), so that an interval of ± 2.5 ◦ C around the optimum comfort temperature is applied. 3. Results and Discussion For the analysis of the monitoring results, data collection corresponding to holidays and non-school days have been removed, while daily measurement data focused on the occupation interval, ensuring minimal distortion of the results. Sustainability 2021,13, 2699 8 of 17 For this study, a representative monitoring week from winter period 1 (before the COVID-19 pandemic) from 23 January to 27 January 2020 was selected. It should be noted that during the pandemic, the issue of classroom air quality was considered a priority over thermal comfort. During a first stage of this period (December 2020), manual airing was mandatory at all times due to COVID-19 protocol and teachers were not authorised to activate the air conditioning systems, so that the level of clothing of the users was higher than usual. However, a sharp drop in temperatures in January 2021 led to a change in control protocols and teachers were granted official permission to control the air conditioning equipment. Therefore, in the case of period 2, the week selected for this research was that from 11 January to 15 January 2021 (control of HVAC systems). However, monitoring results were also analysed between 30 November and 4 December 2020 (no control of HVAC systems). A form was filled out by the teachers of each classroom and ventilation protocols used in each period analysed (before and during the pandemic) were indicated. Table 4summarises the information on the HVAC protocols used in each classroom, during each period. Table 4. HVAC protocols in each period. HVAC Protocols Manual airing Mechanical ventilation and heating Period 1 January 2020 No manual airing Mechanical ventilation + heating 7:30 a.m. to 2:00 p.m. South Class (A5) No manual airing North Class (A6) Manual airing (cross) 9:00 a.m. to 2:00 p.m. Mechanical ventilation + heating 7:30 a.m. to 2:00 p.m. South Class (A5) Period 2a January 2021 Manual airing (one window) 11:30 a.m. to 2:00 p.m. No mechanical ventilation, only heating 7:30 a.m. to 2:00 p.m. North Class (A6) Manual airing (cross) 7:30 a.m. to 2:00 p.m. Neither mechanical ventilation nor heating South Class (A5) Period 2b December 2020 Manual airing (one window) 7:30 a.m. to 2:00 p.m. Neither mechanical ventilation nor heating North Class (A6) 3.1. Thermal Environment Evaluation In this section, the evolution of hygrothermal conditions before and during the COVID- 19 pandemic is analysed, assessing the impact of the variation of the ventilation protocols on users’ thermal comfort. The evaluation takes into consideration the percentage of weekly hours in discomfort, both adaptive and static, for each standard analysed in this study. In light of the results, during period 1, HVAC systems were continuously switched on during the occupation interval and classrooms were only ventilated mechanically, so that outdoor air was heated before being introduced into the classroom. This fact, together with internal loads produced by high density of occupation, results in temperatures of around 15 ◦ C in classrooms at the beginning of the teaching day, but reaching a maximum of almost 30 ◦ C, 12 ◦ C above the maximum outdoor temperature. In this regard, a variation of more than 12 ◦ C in indoor temperature was observed in A5 (south) (Figure 4), as higher solar gains were obtained during the central hours of the day. In period 2, the heating was in operation during all the occupied hours and the ventilation protocol is hybrid, so that classrooms were naturally ventilated during a percentage of the occupied hours. In this case, indoor temperatures always remained below 24 ◦ C, with indoor variations around 7 ◦ C, and a difference of 8 ◦ C above the maximum outdoor temperature. This is the result of the low outdoor temperatures, with a mean value of around 7 ◦ C throughout the week. Higher temperature oscillations were observed in period 1 than in period 2 also due to a dissipation of the thermal loads when the windows were open. It should also be noted that, in both cases, the maximum outdoor temperature was lower than the indoor temperature as the heating was in operation. However, as the average indoor temperature was excessively high during period 1, in the south-facing classroom (A5), this affected the analysis of the comfort conditions. Sustainability 2021,13, 2699 9 of 17 Sustainability 2021, 13, x FOR PEER REVIEW 9 of 17 Figure 4. Comparison of the evolution of hygrothermal conditions between the periods monitored: ( a ) period 1, January 2020, before the COVID-19 pandemic, and (b) period 2a, January 2021, during the COVID-19 pandemic. Sustainability 2021,13, 2699 16 of 17 20. ASHRAE. Thermal Environmental Conditions for Human Occupancy; ASHRAE Standard 55-2017; American Society of Heating, Refrigerating and Air Conditioning Engineers: Atlanta, GA, USA, 2017. 21. AIR. Ausschuss für Innenraumrichtwerte (German Committee on Indoor Guide Values); German Federal Environment Agency: Dessau-Roßlau, Germany, 2019. 22. WHO. Modes of Transmission of Virus Causing Covid-19: Implications for Ipc Precaution Recommendations: Scientific Brief; World Health Organization: Geneva, Switzerland, 2020. 23. Noorimotlagh, Z.; Jaafarzadeh, N.; Martínez, S.S.; Mirzaee, S.A. 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