Indoor CO 2 and Thermal Conditions in Twenty Scottish Primary School Classrooms with Different Ventilation Systems during the COVID-19 Pandemic

: Healthy indoor environments inﬂuence the comfort, health and wellbeing of the occupants. Monitoring the indoor temperature, relative humidity and CO 2 levels in primary schools during the COVID-19 pandemic was mandated by a local authority in Scotland. The aim was to investigate the comfort and safety of the teachers and their pupils. This paper presents the measurements of indoor climate in 20 classrooms in four different primary schools in Scotland. The schools were of different architypes. The classrooms were of different sizes, orientations and occupancy, and had different ventilation systems. Ventilation was achieved either by manually opening the windows, or by a mechanical ventilation system. Indoor air temperature, relative humidity and carbon dioxide (CO 2 ) concentrations were continuously monitored for one week during the heating season 2020/21. Occupancy and opening of the windows were logged in by the teachers. The ventilation rates in the classrooms were estimated by measuring the CO 2 concentrations. On the 20 classrooms of the study, data of 19 were analysed. The results show that four of the ﬁve mechanically ventilated classrooms performed better than natural ventilation, which indicates that opening the windows depended on the customs and habits. Classrooms in naturally ventilated Victorian buildings have the worst average ventilation rate (4.38 L/s per person) compared to the other classrooms (5.8 L/s per person for the more recent naturally ventilated ones, and 6.08 L/s per person for the mechanically ventilated ones). The results of this preliminary study will be used as the basis to ﬁnd ways to ensure adequate ventilation in natural ventilated classrooms.


Introduction
The quality of the indoor environment (IEQ) has been shown to have an impact on the health and wellbeing of workers [1], who mostly complain about poor indoor air quality (IAQ) [2] and inadequate indoor temperature [3]. Schools are not exempt from these issues, being places where pupils and teachers spend hours in an overcrowded enclosed space, often with poor ventilation [4].
Multiple studies [4][5][6][7][8][9][10][11][12][13][14][15] have shown the importance of maintaining appropriate IAQ in classrooms for the comfort, health and wellbeing of both pupils and teachers. The World Green Building Council has published a factsheet about IAQ for schools, and how bad air affects children [16]. According to this report, a 1000 parts per million (ppm) increase above ambient levels of CO 2 has been linked to a 10%-20% increase in days away from school.
Keeping healthy IAQ in schools will help children stay healthier, more focused and more productive [8,17]; it will help reduce health and wellbeing issues among pupils [9,10,18,19] and

Materials and Methods
The study was at the request of a Local Authority in Scotland during the heating season of the COVID-19 pandemic (winter 2020/21) to assess the CO 2 levels, ventilation rates and thermal comfort in their classrooms. The study was performed in four urban primary schools all chosen by the local authority after receiving the approval to participate from the headteachers. The 20 classrooms were selected by the business managers of the schools in which the study was performed. Overall, 15 of the 20 classrooms were naturally ventilated, whereas 5 used a mechanical ventilation system.
The typology of all classrooms is presented in Table 1. School A, having been built in the early 1960s, is poorly insulated [44] and naturally ventilated, with classrooms on the ground floor having double doors opening directly into the playground.
Schools B and D are composed of different buildings built over the years with different ventilation systems.
School C is a typical Victorian school that is naturally ventilated. Figures 1-17 show the locations and the floorplans of the schools (classrooms identified and devices localised by a red dot .) and illustrate the different architypes and ventilation systems of classrooms of the school buildings. Note: The floorplans are for illustration purpose and are not to scale.
Due to the emergency of the situation during the COVID-19 pandemic, the local authority granted access to each school buildings for a period of one week maximum over the heating season (November-December 2020), during which the measurements were performed (November-December 2020). They included the measurements of CO 2 concentration using TinyTag TGE-0010 and TGE-0011 loggers (accuracy ± 50 ppm, ±2% of reading). Other TinyTag TGU-4500 loggers recorded the indoor temperature (−25 to +85 • C) and RH (0% to 95%). All the data loggers were provided by the local authority.
Logging intervals were set to every minute with min, max and actual readings. Sample collection began by setting up all sensors for a fixed start time on their deployment date.
Once in the classrooms, the CO 2 monitors were plugged in to an available power socket and made safe with the logger mounted in the 'occupied zone' at approximately 1.5 m above floor level.
The temperature and RH sensors were located next to the CO 2 monitors, placed either on the teachers' desks or on shelves (as shown on Figure 18). Both monitors were positioned away from direct sunlight, radiators, doors and windows.
At the end of the one week of monitoring, all monitors were retrieved, and the data were downloaded onto the Tinytag Explorer platform via a USB cable. The data were then output to Microsoft Excel for analysis. ent ventilation systems.
School C is a typical Victorian school that is naturally ventilated. Figures 1-17 show the locations and the floorplans of the schools (classrooms identified and devices localised by a red dot .) and illustrate the different architypes and ventilation systems of classrooms of the school buildings. Note: The floorplans are for illustration purpose and are not to scale.             As any indoor environment is impacted by outdoor climatic conditions, the outdoor temperature, RH, wind speed and direction were also studied alongside the internal measured elements. The purpose was to identify potential correlation between the variations of the indoor and the outdoor conditions. The outdoor climate was monitored through the website https://www.wunderground.com/ (last accessed on the 20 November 2021) from different weather stations, located close to the schools monitored. Values were also recorded from a centrally located weather station owned by Edinburgh Napier University, to validate the data from the Wunderground website.

School
Opening of the windows and doors and the occupancy were logged in by the teachers. The average of the peak values of CO 2 concentration measured during school hours was used to estimate the classrooms ventilation rates. Batterman recently reviewed different methods to assess the ventilation rates, using occupant-generated CO 2 [45]. He described four main methods: the steady-state method, the concentration decay, the built-up method and the transient mass balance equation method. Allen and colleagues published a quick five-step guide to evaluate the ventilation rates in classrooms [46], one of which follows the steady-state method described by Batterman.          Logarithmic decay was used in early experiments in an attempt to evaluate room ventilation rates, but the transitory nature of occupancy experienced, combined with a limited range of concentration decay between occupancy events (flushing), gave inconsistent results. A method for mapping time integrated CO 2 evolution through resolving the first-order partial differential equation for a room ventilated at a specific rate was also undertaken but found to be very time intensive. It was thus decided to utilise time integrated metabolic CO 2 mass balancing to establish comparable metrics.
Considering the pros and cons of each method, and assuming the peak CO 2 is indicative of steady-state CO 2 concentration levels, it could therefore be used to estimate the minimum ventilation rate [41]. The CO 2 generation rate per pupil was assumed to be 0.004 L/s and 0.0054 L/s per teacher [47][48][49]. The outdoor CO 2 was assumed to be 420 ppm for the City of Edinburgh, considering the global average atmospheric CO 2 for 2020 was estimated to be 412.5 ppm (NOAA, 2021). Pollutants 2022, 2, FOR PEER REVIEW 11 School case study Road Green Area School D                     Due to the emergency of the situation during the COVID-19 pandemic, the local authority granted access to each school buildings for a period of one week maximum over the heating season (November-December 2020), during which the measurements were performed (November-December 2020). They included the measurements of CO2 concentration using TinyTag TGE-0010 and TGE-0011 loggers (accuracy ± 50 ppm, ±2% of reading). Other TinyTag TGU-4500 loggers recorded the indoor temperature (−25 to +85 °C) and RH (0% to 95%). All the data loggers were provided by the local authority.
Logging intervals were set to every minute with min, max and actual readings. Sample collection began by setting up all sensors for a fixed start time on their deployment date.
Once in the classrooms, the CO2 monitors were plugged in to an available power socket and made safe with the logger mounted in the 'occupied zone' at approximately 1.5 m above floor level.
The temperature and RH sensors were located next to the CO2 monitors, placed either on the teachers' desks or on shelves (as shown on Figure 18). Both monitors were positioned away from direct sunlight, radiators, doors and windows. At the end of the one week of monitoring, all monitors were retrieved, and the data were downloaded onto the Tinytag Explorer platform via a USB cable. The data were then output to Microsoft Excel for analysis.
As any indoor environment is impacted by outdoor climatic conditions, the outdoor temperature, RH, wind speed and direction were also studied alongside the internal measured elements. The purpose was to identify potential correlation between the variations of the indoor and the outdoor conditions. The outdoor climate was monitored through the website https://www.wunderground.com/ (last accessed on the 20/11.2021)from different weather stations, located close to the schools monitored. Values  Due to the emergency of the situation during the COVID-19 pandemic, the local authority granted access to each school buildings for a period of one week maximum over the heating season (November-December 2020), during which the measurements were performed (November-December 2020). They included the measurements of CO2 concentration using TinyTag TGE-0010 and TGE-0011 loggers (accuracy ± 50 ppm, ±2% of reading). Other TinyTag TGU-4500 loggers recorded the indoor temperature (−25 to +85 °C) and RH (0% to 95%). All the data loggers were provided by the local authority.
Logging intervals were set to every minute with min, max and actual readings. Sample collection began by setting up all sensors for a fixed start time on their deployment date.
Once in the classrooms, the CO2 monitors were plugged in to an available power socket and made safe with the logger mounted in the 'occupied zone' at approximately 1.5 m above floor level.
The temperature and RH sensors were located next to the CO2 monitors, placed either on the teachers' desks or on shelves (as shown on Figure 18). Both monitors were positioned away from direct sunlight, radiators, doors and windows. At the end of the one week of monitoring, all monitors were retrieved, and the data were downloaded onto the Tinytag Explorer platform via a USB cable. The data were then output to Microsoft Excel for analysis.
As any indoor environment is impacted by outdoor climatic conditions, the outdoor temperature, RH, wind speed and direction were also studied alongside the internal measured elements. The purpose was to identify potential correlation between the variations of the indoor and the outdoor conditions. The outdoor climate was monitored through the website https://www.wunderground.com/ (last accessed on the 20/11.2021)from different weather stations, located close to the schools monitored. Values

Results
In the following paragraphs, the indoor temperature, the RH and the CO 2 concentrations measured during the monitored weeks as well as the ventilation rates are discussed. The patterned bars on the different graphs show the results for the classrooms with mechanical ventilation. The monitor in the classroom B3 was unplugged shortly after deployment. Therefore, no data were analysed for this classroom.

Link between Outdoor and Indoor Climate (Temperature and RH)
No evident correlation could be found between outdoor conditions recorded and indoor monitored data. Longer periods of sampling might be necessary to verify the trend.
From the returned log sheets and data recorded therein, it was difficult to quantify exactly how often and for how long the window apertures were opened. Mechanisms for automatically recording window opening and room occupancy should be considered for further studies of this nature.

Indoor Climate (Temperature and RH)
This section details the indoor temperature and RH in all the monitored classrooms, alongside the 17 • C temperature threshold and the 40%-60% 'sweet spot' of RH levels.

Indoor Temperature
To assess the indoor temperature of the classrooms according to the 17 • C threshold, the minimum and the average indoor temperature of each classroom are shown on Figure 19. It shows that school A, room A5 (16.4 • C) and school B, room B4 (16.9 • C) recorded an average indoor temperature below 17 • C: Figure 19. Indoor temperature in the classrooms during school hours, with classrooms B1, B4, B5, D1 and D5 having mechanical ventilation.
To highlight the classrooms with lower than acceptable indoor temperatures, Figure  20 shows the classrooms having an indoor temperature below the 17 °C threshold by % of time and in hours.  Table 2 shows the following.
A5 and B4 are either situated on the ground or the top floor, with less potential heat gain from the neighbouring classrooms. To highlight the classrooms with lower than acceptable indoor temperatures, Figure 20 shows the classrooms having an indoor temperature below the 17 • C threshold by % of time and in hours. To highlight the classrooms with lower than acceptable indoor temperatures, Figure  20 shows the classrooms having an indoor temperature below the 17 °C threshold by % of time and in hours.  Table 2 shows the following.
A5 and B4 are either situated on the ground or the top floor, with less potential heat gain from the neighbouring classrooms.  Table 2 shows the following. A5 and B4 are either situated on the ground or the top floor, with less potential heat gain from the neighbouring classrooms.
Both A5 and B4 have lower occupancy compared to the other classrooms of the study: 22 and 25 persons (pupils and teachers), respectively, compared to 31 on average, with less heat gain from their occupants. A5 has one of the largest openable window areas of the study (33 m 2 compared to 11 m 2 on average) with more heat loss through the windows, while B4 is facing northwest (where the wind was predominantly blowing during the study week).
In addition to A5 and B4, three other classrooms had an indoor temperature below the 17 • C threshold for at least 10% of the time when pupils are in class: C1, C2 and A3, which are either on the ground or top floor and are facing West (C1 and C2).
To summarise, for this study, factors influencing the indoor temperature in classrooms are the occupancy and the openable surface areas of the windows, as well as their orientation (North and West) and the location (ground or top floor) in the buildings.

Indoor RH
Whereas there are no regulations for RH in Scottish schools, it has been shown in other indoor environments that levels below 40% are favourable for the spreading of viruses [42,43], and levels above 60% tend to increase the development of mould. Therefore, a threshold of 40%-60% was set for this study.
According to Figure 21, the classroom B1 from school B has average RH levels of less than 40% (39%RH), and room A5 from school A has an RH above 60% (60.9%).
The room B1 recorded lower RH values and had a higher average indoor temperature, above 20 • C as shown on Figure 19, with fewer windows recorded as being open compared to other classrooms in the same school. Depending on outside environmental conditions, lowering the room temperature through increased use of natural ventilation would generally increase relative humidity in these spaces. Figure 22 shows for how long the RH is either less than 40% or more than 60%, while the pupils are in school. The room B1 recorded lower RH values and had a higher average indoor temperature, above 20 °C as shown on Figure 19, with fewer windows recorded as being open compared to other classrooms in the same school. Depending on outside environmental conditions, lowering the room temperature through increased use of natural ventilation would generally increase relative humidity in these spaces. Figure 22 shows for how long the RH is either less than 40% or more than 60%, while the pupils are in school.   The room B1 recorded lower RH values and had a higher average indoor temperature, above 20 °C as shown on Figure 19, with fewer windows recorded as being open compared to other classrooms in the same school. Depending on outside environmental conditions, lowering the room temperature through increased use of natural ventilation would generally increase relative humidity in these spaces. Figure 22 shows for how long the RH is either less than 40% or more than 60%, while the pupils are in school.  In total, 18 of the 20 classrooms monitored had an RH out of the 40%-60% threshold, either being lower or higher, with B4 and D5 having the highest number of hours performing out with the threshold, and both classrooms being mechanically ventilated.

Carbon Dioxide Concentration
The recommended thresholds for room occupancy as provided by Scottish Government advice were 1500 ppm for normal teaching classrooms and 800 ppm for music, drama and gym [35]. Unsurprisingly, the maximum CO 2 levels occurred during school time, when the pupils were present in the class, as it has been shown in other studies [50]. Pollutants 2022, 2, FOR PEER REVIEW 18 In total, 18 of the 20 classrooms monitored had an RH out of the 40%-60% threshold, either being lower or higher, with B4 and D5 having the highest number of hours performing out with the threshold, and both classrooms being mechanically ventilated.

Carbon Dioxide Concentration
The recommended thresholds for room occupancy as provided by Scottish Government advice were 1500 ppm for normal teaching classrooms and 800 ppm for music, drama and gym [35]. Unsurprisingly, the maximum CO2 levels occurred during school time, when the pupils were present in the class, as it has been shown in other studies [50]  School A, where all classrooms have natural ventilation, has never recorded values going above the threshold, indicating adequate ventilation to achieve guideline levels.
Peak values going beyond the thresholds, and their duration at such levels, are an important consideration. However, when the CO2 levels reach very high concentration for very short periods of time (less than 20 consecutive minutes), it might be assumed a change in the physical environment occurred, such as opening of more windows and doors, or changes in occupancy, such as pupils sent outside for breaks. When looking at data in more detail, most of these episodes evidenced CO2 concentrations above the Scottish Government thresholds for only short periods of time and can be put into perspective School A, where all classrooms have natural ventilation, has never recorded values going above the threshold, indicating adequate ventilation to achieve guideline levels.
Peak values going beyond the thresholds, and their duration at such levels, are an important consideration. However, when the CO 2 levels reach very high concentration for very short periods of time (less than 20 consecutive minutes), it might be assumed a change in the physical environment occurred, such as opening of more windows and doors, or changes in occupancy, such as pupils sent outside for breaks. When looking at data in more detail, most of these episodes evidenced CO 2 concentrations above the Scottish Government thresholds for only short periods of time and can be put into perspective when comparing these with classrooms that evidenced CO 2 concentrations above the threshold for more than 20 consecutive minutes (as per regulation in place).
The following school classrooms, representing 42% of all classrooms, were recorded with CO 2 concentrations above the threshold (1500 ppm for normal subjects and 800 ppm for music, drama and gym) for more than 20 consecutive minutes: -School B: rooms B1 (2% above 1500 ppm), B2 (18% of the time above 1500 ppm), B4 (2% of the time above 1500 ppm) and B5 (69% of the time above 1500 ppm); -School C in rooms C1 (1%), C4 (9% of the time above 1500 ppm) and C5 (4%); -School D in room D2 (22% of the time above 1500 ppm). Figure 24 shows the classrooms that have CO 2 concentrations above the thresholds for more than 20 min, and how many hours that represents.
Pollutants 2022, 2, FOR PEER REVIEW 19 when comparing these with classrooms that evidenced CO2 concentrations above the threshold for more than 20 consecutive minutes (as per regulation in place).
The following school classrooms, representing 42% of all classrooms, were recorded with CO2 concentrations above the threshold (1500 ppm for normal subjects and 800 ppm for music, drama and gym) for more than 20 consecutive minutes: − School B: rooms B1 (2% above 1500 ppm), B2 (18% of the time above 1500 ppm), B4 (2% of the time above 1500 ppm) and B5 (69% of the time above 1500 ppm); − School C in rooms C1 (1%), C4 (9% of the time above 1500 ppm) and C5 (4%); − School D in room D2 (22% of the time above 1500 ppm). Figure 24 shows the classrooms that have CO2 concentrations above the thresholds for more than 20 min, and how many hours that represents. Three classrooms (B2, B5 and D2) in two different schools had CO2 levels above the threshold for more than 10% of the occupied time. B2 and D2 are naturally ventilated, while B5 is mechanically ventilated, which can suggest B5 ventilation system is not supplying enough fresh air (hypothesis will be confirmed in Section 3.3.1.2.).
The following paragraphs analyse the potential reasons for these highest concentrations.

Analysis of the CO2 Concentration for B5
The CO2 concentration in B5 (illustrated in Figure 25) was concerning, considering the peak levels were not recorded due to the operating range of the CO2 sensor installed (2000 ppm), assuming the accurate CO2 levels were higher. The space is serviced by a recirculatory HVAC system serviced by a heat pump. When considering the ventilation openings recorded on that same day, it was shown that little to no adventitious ventilation support (through openable windows/louvres) was enabled in the room. The Council has since sent technical staff to the site to evaluate issues. In fact, the windows have full height opening sashes that open inwards and are free to swing once open with no restriction on how far it can open. The windows also have the capability to tilt. However, staff did not have keys to enable these functionalities and therefore relied only on the HVAC system. This issue was raised by the local authority directly with the school. Three classrooms (B2, B5 and D2) in two different schools had CO 2 levels above the threshold for more than 10% of the occupied time. B2 and D2 are naturally ventilated, while B5 is mechanically ventilated, which can suggest B5 ventilation system is not supplying enough fresh air (hypothesis will be confirmed in Section Analysis of the CO 2 Concentration for B5).
The following paragraphs analyse the potential reasons for these highest concentrations.

Analysis of the CO 2 Concentration for B5
The CO 2 concentration in B5 (illustrated in Figure 25) was concerning, considering the peak levels were not recorded due to the operating range of the CO 2 sensor installed (2000 ppm), assuming the accurate CO 2 levels were higher. The space is serviced by a recirculatory HVAC system serviced by a heat pump. When considering the ventilation openings recorded on that same day, it was shown that little to no adventitious ventilation support (through openable windows/louvres) was enabled in the room. The Council has since sent technical staff to the site to evaluate issues. In fact, the windows have full height opening sashes that open inwards and are free to swing once open with no restriction on how far it can open. The windows also have the capability to tilt. However, staff did not have keys to enable these functionalities and therefore relied only on the HVAC system. This issue was raised by the local authority directly with the school. Both classrooms are situated in a Victorian building and are naturally ventilated (Figure 26). Figure 27 shows the temperature and CO2 concentration monitored in B2 over the duration of the study while the school was open for the pupils, alongside the number of occupants and windows openings.
The number of pupils present was almost constant throughout the study days (minimum 26, maximum 27).
The teacher left the maximum windows open during two full days of the week, opening half of the windows for the remaining days.

Analysis of the CO 2 Concentration for B2 and D2
Both classrooms are situated in a Victorian building and are naturally ventilated ( Figure 26). Figure 27 shows the temperature and CO 2 concentration monitored in B2 over the duration of the study while the school was open for the pupils, alongside the number of occupants and windows openings.
The number of pupils present was almost constant throughout the study days (minimum 26, maximum 27).
The teacher left the maximum windows open during two full days of the week, opening half of the windows for the remaining days.
It is noticeable the peaks of CO 2 concentrations build up throughout the week even though the CO 2 concentrations at the start of each day are the same: the levels are higher at the end of the week than at the start, with a peak on the Friday. The lower levels correspond to a break when the children are leaving the classroom (either mid-morning, for lunch or at the end of the school day). This phenomenon may be caused by residual CO 2 from the other part of the building: corridors, halls, etc., where the ventilation strategies are unknown. It is noticeable the peaks of CO2 concentrations build up throughout the week even though the CO2 concentrations at the start of each day are the same: the levels are higher at the end of the week than at the start, with a peak on the Friday. The lower levels correspond to a break when the children are leaving the classroom (either mid-morning, for lunch or at the end of the school day). This phenomenon may be caused by residual CO2 from the other part of the building: corridors, halls, etc., where the ventilation strategies Having more windows open impacts the CO 2 concentration in two ways: the level decreases quicker and takes longer to increase.
Considering the number of pupils in the classroom is almost constant, pupils leaving the classroom has a direct impact on the CO 2 levels: less occupancy implies lower CO 2 levels. Figure 28 shows the CO 2 concentrations and the indoor temperature over the study week for the classroom D2. However, the openings and occupancy were reported accurately for this classroom, which is a limitation, and therefore will not be discussed.
It is noticeable the peaks of CO2 concentrations build up throughout the week even though the CO2 concentrations at the start of each day are the same: the levels are higher at the end of the week than at the start, with a peak on the Friday. The lower levels correspond to a break when the children are leaving the classroom (either mid-morning, for lunch or at the end of the school day). This phenomenon may be caused by residual CO2 from the other part of the building: corridors, halls, etc., where the ventilation strategies are unknown.
Having more windows open impacts the CO2 concentration in two ways: the level decreases quicker and takes longer to increase.
Considering the number of pupils in the classroom is almost constant, pupils leaving the classroom has a direct impact on the CO2 levels: less occupancy implies lower CO2 levels. Figure 28 shows the CO2 concentrations and the indoor temperature over the study week for the classroom D2. However, the openings and occupancy were reported accurately for this classroom, which is a limitation, and therefore will not be discussed. The variation in the CO 2 concentrations is more important for this classroom. It is assumed no pupils came in class on day 4, as very low levels were recorded.
When the classroom is occupied consecutively for full days, the same observation as for B2 can be made: the CO 2 concentrations are getting higher over the days, but when the classroom is empty for a longer period, the concentrations tend to be lower.

Ventilation Rates
As detailed in paragraph 2, the ventilation rate calculation technique does not rely on the decay method. Table 3 shows the average ventilation rates in the classrooms, estimated using the measured peak CO 2 concentrations during school hours.
In total, 80% of the mechanically ventilated classrooms had an ACH above the recommended 2 h −1 , compared to 71% of the naturally ventilated ones. Mechanically ventilated classrooms had a higher ACH except for B5, due to the issue with the HVAC. The high ACH in school A is likely to be a result of the architype of the school building (as detailed in Table 1). It can be assumed the classrooms with mechanical ventilation relied only on the system when the naturally ventilated classrooms had the instructions to open windows and doors as much as possible during the pandemic. This can explain high ACH in some of the naturally ventilated classrooms.
Only the classroom D5 had ventilation rates above the threshold of 10 L/s per person, which suggests that windows and doors should have been open more often.
Except for classroom B5, ventilation rates were lowest where the classrooms were naturally ventilated (average of 4.9 L/s per person) and the highest in the classrooms with a mechanical ventilation system (average of 6.08 L/s per person).
For naturally ventilated buildings, the average of the ventilation rates of Victorian buildings was lower (4.38 L/s per person) compared to more recent ones (5.8 L/s per person). It was assumed that the mechanical ventilation in room B5 underperformed as discussed in Section Maximum CO 2 Levels and % of Time above Threshold.

Discussion
Of the 20 school classrooms monitored and studied, 19 were studied, as 1 had its monitors unplugged.
On the 19 primary classrooms analysed, this study shows that the peaks of CO 2 concentrations are shown to increase during the day and over the week: the later in the day, the higher the peak CO 2 concentration recorded, and the later in the week, the higher the peak concentrations recorded in classrooms.
As previously anticipated and shown in Section 3.3.2, older Victorian naturally ventilated classrooms (in school C and classrooms D2 to D4) have the lowest ventilation rates. As discussed in Section 3.2, the location and orientation of the classrooms within the school building as well as the occupancy have a direct impact on the indoor temperature.
As expected, this study confirms that mechanically ventilated classrooms, if operated properly, perform better in terms of CO 2 concentrations than naturally ventilated ones [51,52].
Therefore, this study can imply some recommendations. Firstly, all mechanical heating and ventilation systems and manually operated windows servicing classrooms should be checked to ensure correct operation.
Secondly, flush the air by opening fully windows and doors at the start and at the end of the school days.
Thirdly, ensure adequate ventilation (windows and doors) during class time to stimulate cross-ventilation.
Fourthly, windows and doors should be open fully during break and lunch time when pupils are leaving the classroom.
Finally, try to reduce the number of occupants per classrooms or promote outdoor activities.

Limitations
Some limitations apply to this study and can be divided into three categories. First, the limitations are related to human behaviour. The equipment was unplugged for short periods of time or for the whole week. Some teachers did not engage in the study or gave imprecise/incomplete information, including classroom D2.
Secondly, the limitations related to the length of the study and the monitors used. The short period of monitoring and of the memory of the data loggers at high resolution enabled only a snapshot of the data. No remote access to the data implied the need to go onsite to upload the results and we only noticed potential issues once they were uploaded.
Finally, due to the transitory nature of occupancy experienced, combined with a limited range of concentration decay between occupancy events (flushing), the often used decay method to calculate ventilation rates gave inconsistent results. Therefore, an adaptation of the steady-state method was adopted.

Further Development
Following this study, some further development should be envisaged. More research studies on the impact of RH in schools should be considered. Primary school classrooms of Victorian buildings have been shown to have the lowest ventilation rates. A longer and more in-depth study targeting these schools would help confirm the trends found during this study.
Measuring the CO 2 concentrations levels alongside the tVOCs may confirm a correlation already noticed during other studies performed in naturally ventilated classrooms [53].
In that context, raising awareness of the importance of IAQ in schools, to teachers, pupils, headteachers and janitorial staff should enable them to grasp the importance of good indoor air quality. In addition, providing each classroom with visual feedback from monitors for occupants to have the option to act could empower them to feel more in charge of their indoor environment. The use of wireless sensors with remotely accessible data will enable the monitoring to solve any issues as soon as they arise.