*3.2. CO2 Concentration*

The results of the measurements of CO2 concentration during opening hours in the summer and winter are presented in Figure 7. Tables A5 and A6 show that the median CO2 concentrations calculated for opening hours on weekdays were 922 ppm in the summer and 990 ppm in the winter. According to the results of a survey of nursery schools in Denmark [23], the median CO2 concentration during opening hours was 579 ppm, which was lower than that of the nursery classrooms in this study. On the other hand, according to the results of a survey of nursery schools in France [24], the mean CO2 concentration during opening hours was 1200 ppm, which was higher than that of the nursery classroom in this study. In both summer and winter, the mean CO2 concentration in one classroom was found to be over 1500 ppm, which was specified by the school environmental health standard [2]. The mean CO2 concentration during opening hours in the summer exceeded 1500 ppm in the 0-year-old room in nursery K, probably because of a lower ventilation rate, despite meeting the area standard per infant, as shown in Table 2. The mean CO2 concentrations during opening hours in the winter were over 1500 ppm in the 1-year-old room in nursery M, where the ventilation rate was probably low because it did not meet the area standard per infant as mentioned earlier.

Figure 7a shows that during opening hours in the summer, of the 26 measured classrooms, one classroom had a mean value of more than 1500 ppm, and the mean values exceeded 1000 ppm in six classrooms, which is 23% of the measured classrooms. Figure 7b shows that during opening hours in the winter, of the 26 measured classrooms, one classroom had a mean value of more than 1500 ppm, and the mean values exceeded 1000 ppm in eight classrooms, which is 31% of the measured classrooms. In this way, the CO2 concentration in the nursery classrooms tended to be higher in the winter than in the summer. Additionally, except for some nursery schools (nursery I in summer and nursery D, I, and K in winter), there was a significant difference in CO2 concentration between the 0-year-old room and 1-year-old room on weekdays in both the summer and winter. In the summer and winter (nurseries B and K), in the summer (nurseries E, F, and N), and in the winter (nursery J), the CO2 concentration in the 0-year-old room was significantly higher than that in the 1-year-old room. In other nursery schools, the 1-year-old room was found to exhibit a significantly higher CO2 concentration than the 0-year-old room (Tables A5 and A6). The

high concentrations of CO2 occurred when numerous people gathered in one classroom at the time of pick-up or during activities according to its fluctuations (data not shown). However, there was no relationship between CO2 concentration and the areas per infant for both classrooms (Tables 1 and 2).

**Figure 7.** CO2 concentrations in the nursery classrooms during opening hours in (**a**) summer and (**b**) winter.

Figure 8 shows the evaluation results of the IAQ during the opening hours for each nursery classroom in the summer and winter using ICONE [22]. No extreme air stuffiness (ICONE score = 5) was found in any of the nursery classrooms. Of 52 studied classrooms in the summer and winter, only one classroom (2%) had very high air stuffiness (ICONE score = 4), three (6%) had high air stuffiness (ICONE score = 3), 16 (31%) had average air stuffiness (ICONE score = 2), 20 (39%) had low air stuffiness (ICONE score = 1), and 12 (23%) had no air stuffiness (ICONE score = 0). With the exception of some nursery schools, the 0-year-old rooms and 1-year-old rooms had lower percentages (8%) of high air stuffiness (ICONE score ≥ 3) compared with that of nursery schools in France (38% [24]). Both 0-year-old and 1-year-old rooms in nursery K exhibited high or very high air stuffiness scores in both rooms in the summer, and the 1-year-old room in nursery M exhibited high scores in both summer and winter. Table 2 showed that in the 1-year-old room of nursery M, the area per infant did not meet the minimum standard, and the ventilation rate was insufficient due to the high density of the classroom. In nursery K, the small room volume contributed to the insufficient ventilation rate, even though the area per infant met the minimum standard.

**Figure 8.** ICONE air stuffiness index scores in the studied nursery rooms during opening hours in summer and winter.

Figure 9 shows the relationship between the infants' room area and weekly median CO2 concentration in the summer and winter. In the winter, there is a moderate correlation between the area per infant and the weekly median value of CO2 concentration. It can be seen that in the winter, the larger the area per infant, the lower the weekly median CO2 concentration, although there is no similar correlation in the summer.

**Figure 9.** Relationship between infants' room area and weekly median CO2 concentration in (**a**) summer and (**b**) winter.

#### *3.3. Particulate Matter*

The results of PM2.5 measurements during the 10 min spot measuring period in nursery classrooms and outside are presented in Table 4, which shows the number of small particles with a diameter of less than 0.5 μm, indoor-to-outdoor ratios (I/O ratios) that represent the ratios of the number of small particles between the indoor air and outdoor air, and IAQ determined based on the number of the small particles during measuring period in the summer and winter. I/O ratios can determine the impact of indoor/outdoor sources on indoor environments. The small particles were detected in all of the studied classrooms. IAQ in each nursery classroom was rated to be fair, poor, or very poor in most nursery schools, according to an IAQ judgment criteria based on the number of small particles. The air quality criteria were evaluated as follows on a five-point scale according to the number of small particles: 0–75, excellent; 75–150, very good; 150–300, good; 300–1050, fair, 1050–3000, poor; and >3000: very poor. Of the 24 classrooms measured in both summer and winter, 12 had a lower IAQ in the winter than in the summer, five had a lower IAQ in the summer than in the winter, and there was no change in IAQ between summer and winter in the other seven classrooms. The time that the windows are open tends to be shorter in the winter than in the summer, and the lack of ventilation affects the low IAQ result based on the number of small particles. I/O ratios ranged from 0.11 to 4.47 in 0-year-old rooms, and ranged from 0.10 to 2.01 in the 1-year old rooms, throughout the year. Compared by season, the I/O ratios ranged from 0.48 to 4.47 in 0-year-old rooms and ranged from 0.39 to 2.01 in 1-year old rooms in the summer, though the I/O ratios ranged from 0.11 to 1.89 in 0-year-old rooms and from 0.10 to 1.21 in 1-year-old rooms in the winter. In 1-year-old rooms, the I/O ratio was significantly higher in the summer than in the winter (*p* < 0.05), but there was no significant seasonal difference in the 0-year-old rooms. From the viewpoint of I/O ratio, the number of the small particles in the classroom was larger than outside in seven out of 26 classrooms (one-third of the measured rooms) measured in the summer and four out of 26 classrooms (one-sixth of the measured rooms) measured in the winter. In these 11 classrooms in both summer and winter, there is a source of small particles in the classroom. It is believed that small particles detected indoors were influenced by activities such as playing with sand in the garden. Children might come and go more frequently when playing outside in the playground in the summer than in the winter; hence, it may be easier to bring dust from the outside air into the classroom. In a Portuguese preschool environment, values of I/O ratios showed that a considerable part of small particle matter, including PM2.5, originated indoors and carcinogenic risks due to exposure indoors were 10 and 4 times higher than for exposure outdoors, respectively, for younger (3 years) and older (4–5 years) pupils [25]. This was due to the fact that younger children tended to stay indoors for longer periods of time [25]. Therefore, focusing on the health effects of small particles in the classroom is necessary.




**Table 4.** *Cont.*

#### *3.4. Illumination*

Figure 10 shows the mean values of illumination in the studied nursery classrooms during opening hours in the summer and winter, including 95% confidence intervals. It shows a fluctuation from the lighting being temporarily turned off at any time from 12:00 to 14:00 due to the children napping in the nursery classroom, and the illumination is reduced to the minimum (results not shown). Notably, the measurement location may be partially affected by solar radiation due to the convenience of measurement installation. The numbers of the nursery classrooms where the average illumination during opening hours was less than the standard value of 300 lx established by the school environmental health standard [2] were eight (31% of the total) out of the 26 studied classrooms in the summer, and nine out of 26 (35% of the total) in the winter. There was no significant difference in the average illumination between summer and winter. The average illumination in the two classrooms was lower than 100 lx. In particular, the average illumination of the 1-year-old room in nursery F in the summer was under 77 lx, which was significantly lower because the lights were turned off to save energy for space cooling. Infants do not study as do school students, but in consideration for the needs of infant visual development, they do need to maintain minimal illumination for reading picture books and other indoor activities in the classrooms.

**Figure 10.** Mean values of illumination in the studied nursery rooms during opening hours in summer and winter, including 95% confidence intervals.

#### *3.5. Noise Level*

In this study, for nursery schools that could not be measured during naps, the noise levels during various activities were measured and treated as reference values. Results provide evidence showing how noisy it is; noise in the vicinity of nursery schools in Japan is considered to be a social problem. Strictly speaking, the noise level during nap time is not equal to the background noise level, but perhaps close to background noise and therefore useful. The equivalent noise level during various activities ranged from 50.4 to 70.6 dBA with an average of 60.1 dBA in 0-year-old rooms, and ranged from 48.9 to 84.3 dBA with an average of 64.1 dBA in 1-year-old rooms (results not shown). The equivalent noise level during the nap time ranged from 50.4 to 59.3 dBA with an average of 54.4 dBA in 0-year-old rooms and ranged from 49.3 to 58.5 dBA with an average of 53.1 dBA in 1-year-old rooms. According to the guidelines of WHO, the background equivalent noise level in classrooms of preschools should not exceed 35 dB Laeq during class [34]. Compared to this, the noise level in Japanese nursery classrooms is considerably higher even during naps. Considering the development of hearing of infants, establishing appropriate noise standards for nursery school classrooms is necessary.

#### *3.6. Examination of Indoor Environmental Standards*

Indoor environmental standards for nursery school classrooms where children are under 2 years old and who cannot easily wear a mask, cannot receive a COVID-19 vaccination, and spend a long time, will become increasingly necessary in Japan in the future. From the results of this study, first, concerning the thermal environment, the temperature should be 26–28 ◦C in the summer and 20–23 ◦C in the winter, as given in the infectious disease control guidelines for nursery schools [3]. Considering the time that children spend at lower heights close to the floor, this study's findings recommend assessing whether the temperatures at the heights of 0.1, 0.3, and 0.6 m above the floor meet the standard value. If the nursery school buildings are old, their insulation is also important.

Second, regarding relative humidity, the infectious disease control guideline only stipulates a relative humidity of 60% [3], but it is believed that the 40–70% standard with a range applied to office buildings under the Act on Maintenance of Sanitation in Buildings [35] should be applied to nursery school classrooms.

Third, regarding the indoor air environment, it became clear that the CO2 concentrations in the nursery classrooms are high during activities or the waiting time for pick-up; hence, it is proposed that the daily average concentration of CO2 should be less than 1000 ppm and the maximum concentration should not exceed 1500 ppm, with reference to BB101 established in the UK [7], though the infectious disease control guidelines do not set

ventilation standards [3]. Additionally, adhering to the standard for the area of the nursery classroom per infant, which is specified in the minimum standard for child-welfare-facility minimum criteria [28], will lead to satisfying the required ventilation rate per child, and provide a well-ventilated indoor air environment. As for PM2.5 concentration, as mentioned earlier, the spot measurement was only for 10 min; hence, a concrete standard could not be proposed in this study.

Fourth, regarding the illumination environment, a minimum illumination of 300 lx, which is specified in BB101 [7], is required for infants' visual development. The nursery school classrooms are used by people of all ages, including infants' grandparents who pickup the children instead of working parents. Older adults need twice as much illumination as adults [36]; hence, they need more than 300 lx from the viewpoint of universal design.

Fifth, regarding the acoustic environment, the indoor background noise level in the nursery school classrooms should ideally be restricted to 35 dBA, as determined by WHO. According to the results of inspections of nursery facilities in Munich, Germany, where regulations of acoustic design for daycare facilities exist, all the classrooms in the inspected facilities were equipped with sound absorptive material for sound insulation and the reduction in reverberation [37]. By contrast, in Japan, without any standards or regulations, meeting these strict requirements is difficult for nursery schools. Therefore, firstly, as stipulated in the school environmental health standard [2], fulfilling the minimum indoor background noise level of 50 dBA when windows are closed is necessary; however, establishing the indoor equivalent noise level slightly lower to consider the development of hearing in infants is also vital.

#### **4. Conclusions**

The measurements of indoor environments and questionnaire surveys in 15 different nursery schools in mild climatic areas in Japan were conducted in the summer and winter of 2016–2019. The results of the measurements were summarized and the indoor environmental standards that should be set in the nursery school classrooms were considered.

The study revealed that summer mean temperature was between 26 and 28 ◦C, but the winter mean temperature was lower than the lower limit of 20–23 ◦C. Mean RH was close to 60% of the standard in the summer, but lower than the standard in the winter, compared to the infectious disease control guideline. In addition, in the summer, there was one nursery school where the temperature in the 1-year-old room at any height was higher than 30 ◦C at almost time during the day in the summer, which was too high and did not meet the guidelines, although the temperature was lower compared to the lower limit of the guideline in most of the other nursery classrooms. It is necessary to use air conditioning in this nursery school to prevent heat stroke. In order to avoid an extremely hot indoor environment in the summer, the necessity of indoor environmental standards in the nursery classroom was shown. In the winter, it was found that the temperature at 1.1 m above the floor generally met the infectious disease control guidelines for nursery schools (20–23 ◦C), except for some nursery schools such as those in older buildings, in contrast, the temperature at 0.1 m above the floor in all of the nursery classrooms did not meet the standards (20–23 ◦C). This may be due to poor insulation performance or poor air circulation, even in nursery schools that were built more recently. Since infants spend their time at lower heights than adults, it is necessary to raise the temperatures at 0.1, 0.3, and 0.6 m from floor surface in the winter to ensure the comfort and health of infants. It can be said that efforts such as improving the insulation performance of buildings and repairing the insulation of existing buildings are also necessary to solve the problems of lower temperature at the lower height from the floor surface in the nursery classrooms during winter.

The mean RH satisfied the value of 30–80% specified by the school environmental health standard in all studied nursery classrooms both in the summer and winter. The mean HR in most of the studied nursery classrooms did not satisfy the upper limit of 0.012 kg/kg (DA) in the summer, specified by ASHRAE Standard 55-2017, although the mean HR in all studied nursery classrooms satisfied the requirement in the winter. Since it is hot and humid during the summer in Japan, focusing on humidity control in the summer is important from the viewpoint of microbial contamination.

Nursery school classrooms were affected by cold radiation because the globe temperature differences were negative in most classrooms both in the summer and winter.

The mean CO2 concentrations during opening hours exceeded 1500 ppm, specified by the school environmental health standard, in one classroom both in the summer and winter. The reasons for this were probably because of the low ventilation rate despite meeting the area standard per infant or insufficient ventilation rate because of not meeting the area standard per infant. The evaluation results of the IAQ during the opening hours of each nursery room in the summer and winter using ICONE showed that the 0-year-old and 1-year-old rooms, except some nursery schools, exhibit lower percentages (8%) of high air stuffiness (ICONE score ≥ 3).

Small particles with a diameter of less than 0.5 μm were detected in all studied classrooms. IAQ in each nursery classroom was rated to be fair, poor, or very poor in most nurseries, according to an IAQ judgment criteria based on the number of small particles. Half of the nursery school classrooms exhibited lower IAQ in the winter than the summer. The opening time of the windows is probably shorter in the winter than in the summer, and the lack of ventilation affects the low IAQ in the winter. As a result of I/O ratios, the number of small particles in the classrooms was larger than outside in one-third of the studied classrooms in the summer and one-sixth of the studied classrooms in the winter; therefore, sources of small particles may be present indoors. It is believed that small particles detected indoors were influenced by activities such as playing with sand in the garden.

The mean illumination during opening hours in one-third of the studied nursery classrooms was less than the standard value of 300 lx established by the school environmental health standard—both in the summer and winter. Considering the needs of infant visual development, minimal illumination for indoor activities for infants should be maintained.

The equivalent noise level by applying the A-weighting filter in studied nursery classrooms was considerably high even during nap times. Therefore, considering the development of infant hearing, the need for an acoustic standard for nursery school classrooms was shown.

To summarize, some nursery school classrooms were found to have large vertical temperature differences, while other classrooms had poor IAQ because they held more children than their capacities allowed. Compliance with the specified area per child would lead to the maintenance of a desirable IAQ. In this study, the need for indoor environmental standards in nursery schools was strongly indicated in terms of infants' comfort and health. It is necessary to continue to evaluate the actual conditions of indoor environment in nursery school classrooms and establish suitable indoor environment standards as soon as possible. Future indoor environmental standards for nursery classrooms—where children under 2 years old spend considerable time, who cannot wear a mask easily, and cannot receive COVID-19 vaccinations—will become increasingly necessary in Japan.

### **5. Limitations to the Study and Future Research**

This study had the following limitations:


(4) In this study, indoor air environment was evaluated by CO2 concentration and particulate matter, but investigating the ventilation rate for each nursery classroom will be vital in the future.

**Funding:** This research was partially funded by LIXIL JS Foundation in 2016, grant number 16-13.

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Acknowledgments:** I deeply appreciate the assistance of all the people involved in the nursery schools who cooperated in the survey. I am extremely grateful to all the students at Nagasaki University who supported me throughout the preparation and administration of the survey.

**Conflicts of Interest:** The author declares no conflict of interest.

#### **Appendix A**

**Table A1.** Vertical temperature during opening hours in summer.



**Table A2.** Vertical temperature during opening hours in winter.

**Table A3.** Relative humidity, humidity ratio, and globe temperature during opening hours in summer.



**Table A3.** *Cont.*

**Table A4.** Relative humidity, humidity ratio, and globe temperature during opening hours in winter.



**Table A4.** *Cont.*

**Table A5.** CO2 concentrations (ppm) in nursery classrooms during opening hours in summer.



**Table A6.** CO2 concentrations (ppm) in nursery classrooms during opening hours in winter.

#### **References**

