The Effect of Avoiding Closed Spaces on the Indoor Environment of Nursery School Rooms during COVID-19 winter

. This study aimed to clarify the effect of the policy of avoiding closed spaces with poor ventilation for preventing COVID-19 on the indoor environment of nursery school rooms during winter. The measurement survey was conducted in 23 rooms of nine nursery schools in the Tokai region of Japan. We measured the CO 2 concentration, temperature, and humidity in 10 min intervals during the winter in 2022. The data collected from 9 a.m. to 3 p.m. on weekdays when the rooms were occupied were analyzed. The results showed that the mean value of the CO 2 concentration in the nursery school rooms was below 1000 ppm in 22 of 23 rooms. Besides, we estimated the ventilation rate and found that most rooms exceeded the value per person required to keep the concentration below 1000ppm. The results suggest that the nursery school teachers actively used mechanical and window-opening ventilation. On the other hand, the mean temperature was below 18 °C in 12 rooms and the mean relative humidity was below 40% in 22 rooms. We found that increasing the ventilation volume to avoid closed spaces during winter significantly decreased the relative humidity in the nursery school rooms.


Introduction
In December 2019, COVID-19 infections began in Wuhan, China, and spread to many countries worldwide in the following February. The Omicron strain of infection increased in both children and adults. As of November 2022, certain schools and nurseries remain closed.
There has been some research on coronavirus infections and ventilation methods for improving the indoor air environment in schools. Rodriguez et al. [1] conducted a field survey on indoor CO2 levels in secondary schools and universities during the COVID-19 pandemic. They found that controlled mechanical ventilation systems are essential in educational spaces, and that wearing a properly fitting FFP2-N95 mask indoors should also be strongly recommended to minimize the risk of airborne infection transmission. Barrio et al. [2] measured indoor temperatures and CO2 concentrations to compare indoor environmental conditions before and during the COVID-19 pandemic in winter in public middle and high schools. The results showed that the mean CO2 concentration decreased by 1,400 ppm, with a second term value of approximately 1,000 ppm, as the temperature decreased by nearly 2 °C, with a mean value of 18 °C.
In Japan, the New Coronavirus Infectious Disease Control Subcommittee summarized effective ventilation methods for preventing the spread of COVID-19 in * Corresponding author: 2022y03@stu.gifu-nct.ac.jp buildings [3]. The Ministry of Health, Labor, and Welfare [4] has established guidelines for infectious disease control in nursery schools. However, few studies based on field surveys in nursery schools have been conducted.
Therefore, this study aimed to clarify the effect of the policy of avoiding closed spaces with poor ventilation for preventing COVID-19 on the indoor environment of nursery school rooms during winter.

Method
We selected nine nursery schools in the Tokai region of Japan for this study. A measurement survey was conducted in 23 rooms regularly used by the children. The indoor carbon dioxide (CO2) concentration, temperature, and relative humidity was measured in 10 min intervals during winter for approximately 40 days from mid-January to late February 2022. A CO2 logger (TR-76Ui; accuracy: ±50ppm T&D Corp.) was used to measure the CO2 concentration, and a temperature/humidity logger (TR-72U; accuracy: ±0.3°C, ±5%, T&D Corp.) was used to measure the temperature and relative humidity in each room. Table  1 lists the structure of the facilities, year in which they were built, number of children, floor space, and presence or absence of a humidifier in the room. The temperature and humidity of the outside air was obtained from the Automated Meteorological Data Acquisition System (AMeDAS) weather data of the same city. According to the Japan Meteorological Agency, the standard climate values of temperature, humidity, and maximum snow accumulation in the prefectural capitals in this region are 5.3 °C, 62.3%, and 4.5 cm, respectively. Furthermore, nursery teachers were interviewed regarding the temperature settings, ventilation methods, and room occupation times. Based on these interviews, the data collected from 9 a.m. to 3 p.m. on weekdays when the rooms were regularly occupied were analyzed.

Box-and-whisker diagram
Japan's Ministry of Health, Labour and Welfare has indicated that if the concentration of carbon dioxide is 1000ppm or less, it is not a closed space with poor ventilation. All of the nursery rooms surveyed used air conditioners as heating equipment. Open stoves were used in some rooms as a supplement, but only temporarily in the morning. Fig. 1 shows a box-and-whisker diagram of carbon dioxide concentrations. The notches on the box plots indicate a 95% confidence interval of the median calculated, and the cross mark indicates the mean value. The mean value in nursery rooms was below 1000ppm in 22 out of 23 rooms except for Tc1. Furthermore, the third quartile values were below 1000 ppm in 22 rooms. In particular, Th1 had a small quartile range and was close to the outdoor air concentration. Mechanical ventilation equipment was installed in 12 of the 23 rooms, but they were not of the heat exchange type. In interviews with each facility, they answered that most rooms were ventilated by opening the windows. Therefore, we found that many rooms are sufficiently ventilated during winter regardless of whether they have ventilation equipment or not.
On the other hand, in Tc1, the mean value exceeded 1000 ppm and there was a wide range of scatter above and below the median. According to the interviewees, they could not open the windows in this room because the noise from inside the room could be heard through the open windows and disturbed the neighbors. However, the constant use of mechanical ventilation equipment suggested that the equipment lacked ventilation capacity.

Time of day when the concentration tends to increase
Next, we tried to reveal the times and activities at which CO 2 concentrations were likely to rise. Daily activities often include morning homeroom, indoor and outdoor play, lunch, nap, and afternoon meeting. The mean daily variation was determined by averaging each measurement at each time, and the peak time of concentration was calculated. The peak times for each room are shown in Fig. 2. The peak CO 2 concentration times varied: some rooms had peak times around 9 a.m. and some rooms had peak times after 1 p.m. Reasons given for peak CO 2 concentrations in the morning included the following Everyone was gathered in the room for the morning meeting.
Temporary use of open oil stove due to low temperatures in the morning.
On the other hand, the following reasons were considered for rooms with peak concentrations in the afternoon.
The concentration gradually increased from the morning and reached its peak before the children left the nursery school.
Window opening ventilation was avoided during afternoon naps. From the above, it was inferred that the time of day when the concentration tends to be high in each room is different and that the activity conditions are also different.

Ventilation rate
Furthermore, we regarded the time under consideration to be in a steady state and thus estimated the ventilation rate from actual measurements. The required ventilation rate needed to be maintained below a concentration of 1,000 ppm using the Seidel formula (Eq.1). 2 concentration in the room, and po (m 3/ m 3 ) is the outside CO 2 concentration. The CO 2 exhalation was set at 0.011 and 0.022 m 3 /h for a child and nursery school teacher, respectively, based on the Standard for School Environmental Sanitation [5]. The outdoor CO 2 concentration was set at 423 ppm based on data from a meteorological observatory. This value is the mean value for January and February observed in Ayasato. Fig.  3 shows the estimated and required ventilation rates. Incidentally, the rates were not shown in terms of ventilation frequency because many ceiling shapes are difficult to calculate in terms of volume owing to the diversity of nursery room designs.

where (m 3 /h) is the ventilation rate, k (m 3 /h) is the pollutant emissions, pi (m 3 /m 3 ) is the acceptable CO
As shown in Fig. 3, the minimum and maximum estimated ventilation rates were 326 and 2929 m 3 /h in Tc1 and Td2, respectively. In contrast, the minimum and maximum ventilation requirements, which were below 1,000 ppm, were 210 m 3 /h in and 877 m 3 /h in Ta2 and Te2, respectively. Fig. 4 shows the Fig. 3 results converted to an estimated ventilation rate per person (child). The Japanese Ministry of Health, Labor, and Welfare and HASS102 (SHASE of Japan) recommend a maximum ventilation rate of 30 m 3 /h. This is the value for an adult. With the exception of Tc1 and Ti1, the rooms all exceeded 30 m 3 /h. More than 60 m 3 /h was observed in eight of the 23 rooms. This indicates that excessive ventilation by opening windows may have a significant impact on the indoor temperature and humidity during winter.

Box-and-whisker diagram
Box-and-whisker plots of the indoor temperatures are shown in Fig. 5. The mean temperature was below 18 °C in 12 of the 23 rooms. The temperature range recommended by the Guidelines for the Control of Infectious Diseases in Daycare Centers is 20-23 °C. The mean temperature was within this range in only 2 of the 23 rooms, and included Td, which had the highest CO2 concentration. Box-and-whisker plots of the indoor relative humidity are shown in Fig. 6. Only 1 of the 23 rooms had a mean temperature above 40% and 10 rooms had mean temperatures below 30%. These results indicate that the promotion of ventilation may result in a decrease in temperature and humidity.  Ta1  Ta2  Ta3  Tb1  Tb2  Tc1  Tc2  Tc3  Td1  Td2  Te1  Te2  Te3  Tf1  Tf2  Tf3  Tg1  Tg2  Tg2  Tg3  Th1  Ti1  Ti2  Ti3 Ventilation rate [ m 3  Ta2 Ta3  Tb1  Tb2  Tc1  Tc2  Tc3  Td1  Td2  Te1  Te2  Te3  Tf1  Tf2  Tf3  Tg1  Tg2  Tg2  Tg3  Th1  Ti1  Ti2  Ti3 Vnetilation  Fig. 7 shows the relationship between the mean CO 2 concentration and mean indoor temperature. Although there was a slight tendency for the room temperature to decrease as the CO 2 concentration decreased, the correlation coefficient was as low as r = -0.14. We supposed that as the effect of ventilation was small in rooms with a high heating capacity, the indoor temperature could be maintained. Fig. 8 shows the relationship between the mean CO 2 concentration and mean indoor relative humidity. As the CO 2 concentration decreased, the relative humidity also tended to decrease. The correlation coefficient was r = 0.51. The regression line shows that the relative humidity falls below 40% when the concentration falls below 1000 ppm. Moreover, the regression line also shows that the relative humidity was below 30% when the CO 2 concentration was lower than about 620 ppm. Similar results were obtained even when Tc1, which had a high concentration, was removed. Fig. 9 shows the relationship between the mean CO 2 concentration and indoor/outdoor absolute humidity difference. From the whole regression line, the indoor absolute humidity tended to decrease as the CO 2 concentration decreased. In particular, in some rooms where the indoor/outdoor difference was approximately 0, the absolute humidity was almost the same as the outdoor air, thus dry. Besides, we conducted a t-test and found that only four rooms absent humidifiers showed no trend toward significantly higher absolute humidity.

Relationship between CO2 concentration and temperature or humidity
Therefore, if the heating capacity of the room is high, the indoor temperature is not significantly reduced by ventilation. However, the indoor humidity is significantly reduced by ventilation, and this drying is concluded to be a problem.

Conclusion
CO 2 concentrations in most of the nursery rooms were below 1000 ppm during winter owing to ventilation promotion to prevent the spread of COVID 19. Besides, some rooms were significantly above the required ventilation rate per person to remain below 1000 ppm. We found that in nursery rooms, since opening windows are mainly used for ventilation and the ventilation equipment does not have a heat exchange, excessive ventilation leads to low temperature and humidity. Therefore, we considered that a ventilation facility having a total heat exchanger is effective, but excessive ventilation must be avoided first.
Moreover, since the Tokai region targeted for this survey belongs to the Pacific climate, we presume that the above results were influenced by a warm and dry climate. In the future, it is necessary to consider the existence of regionality.