Assessment of Perceived Indoor Air Quality in the Classrooms of Slovenian Primary Schools and Its Association with Indoor Air Quality Factors, for the Design of Public Health Interventions

Abstract

From a public health perspective, it is necessary to improve indoor air quality (IAQ) in schools. This study aims to assess the state of perceived IAQ in Slovenian school classrooms and its association with the selected IAQ factors to improve the understanding of perceived IAQ for designing public health interventions aimed to improve IAQ in schools. A national cross-sectional study was performed in all 454 Slovenian primary schools in the school year 2019/2020. The questionnaires were filled out by the 3rd-grade teachers with the support of the caretakers. Teachers rated the IAQ in the classroom as the worst in winter. We found that the teachers’ perceived IAQ in the classroom is statistically significantly associated with the micro location of the school and some of the IAQ factors. Poor IAQ is associated with reduced manual airing of classrooms due to the thermal comfort of the occupants. Interventions should be aimed at improving occupants’ adaptive behaviors to increase the frequency of natural ventilation in classrooms.

1. Introduction

Poor indoor air quality (IAQ) is a growing problem in both developing and developed countries. According to recent estimates, indoor air pollution represents the ninth-largest burden of disease worldwide [1]. Due to the small volume of indoor air and many different sources of pollution, the concentration of pollutants in indoor air is higher than in outdoor air [2]. Poor IAQ is associated with several acute, as well as chronic, symptoms and diseases, of which respiratory and cardiovascular effects are the most important [3]. The World Health Organization identifies benzene, carbon monoxide, formaldehyde, naphthalene, nitrogen dioxide, radon, trichloroethylene, tetrachloroethylene, and polycyclic aromatic hydrocarbons as the most important indoor air pollutants with an impact on human health [4]. Some of these pollutants are also carcinogenic, with radon being classified as a Group I human carcinogen by the International Agency for Research on Cancer [5]. In addition, exposure to indoor moisture and mold is clinically associated with respiratory symptoms and can cause a variety of hypersensitivity reactions [6]. Children and adolescents spend up to 8 h per day in educational settings, representing approximately one-third of their daily exposure [7]. Poor IAQ in educational settings is associated with a negative impact on school childrens’ learning productivity and educational performance [8,9], as well as their absenteeism [10,11].

The predominant climate in Slovenia, where we conducted our study, is temperate continental [12]. According to the national cross-sectional study, almost half of Slovenian primary schools are located in a village/rural area. Almost half of the buildings of Slovenian primary schools were built between 1960 and 1979, while 80% of all primary schools have renovated their buildings in the last 20 years. Due to the numerous renovations over the last 20 years, the age of the building does not have a major impact on classrooms IAQ [13].

From a public health perspective, it is necessary to improve IAQ in schools; 86.96% of 3rd-grade classrooms in Slovenian primary schools are ventilated with natural ventilation, 4.04% with mechanical ventilation, and 9.00% with hybrid ventilation [14]. As the majority of the classrooms in Slovenian schools are naturally ventilated via manual airing (i.e., natural ventilation by window opening), the IAQ is not properly managed, neither is the adequate supply of fresh air to the classroom, which is normally achieved with mechanical ventilation. A comparative analysis of IAQ in primary school classrooms with different types of ventilation showed that classrooms with natural ventilation tend to have the worst IAQ [15]. In school classrooms, the IAQ is normally not monitored via continuous measurements that would provide relevant information regarding potential elevated concentrations of indoor air pollutants. According to studies [16,17,18,19,20,21], a possible approach to identify an inadequate indoor environment in educational settings may include the perceived IAQ, which shows certain correlations with the occurrence of elevated levels of airborne compounds, such as CO₂, PM_2.5, VOCs, molds, bacteria, and respirable dust. The studies [18,19,20] also suggest that poor perceived IAQ in the classroom may have an indirect association through elevated air pollutant levels on school childrens’ and teachers’ health. Therefore, it is valuable to collect data directly from the classroom users in order to improve the IAQ in Slovenian schools. Furthermore, at the international level, post-occupancy evaluation is one of the methods used to determine the factors that affect the indoor environment [22]. In this regard, it is important to consider that classroom IAQ is affected by a number of factors, both indoor and outdoor [13].

In order to design interventions to improve IAQ in schools, it is important, among other things, to consider the experience of users, which can be presented by the perceived IAQ in classrooms. Therefore, this study aims to assess the state of perceived IAQ in Slovenian school classrooms and its association with the selected IAQ factors to improve the understanding of perceived IAQ for designing public health interventions aimed at improving the IAQ in schools.

2. Materials and Methods

This study was conducted as part of the ARIS Research Project (No. V3-1904) and was approved by the Medical Ethics Committee of the Republic of Slovenia (No. 0120-548/2019/4). In the first research phase, we developed a validated questionnaire and identified and assessed the prevalence of IAQ factors in Slovenian primary schools and their association with the micro location and year of construction of the school building [13]. In this study, we present the second research phase: perceived IAQ and its association with the IAQ factors in classrooms of Slovenian primary schools.

2.1. Study Design, Population, Observation Period, and Area

The national cross-sectional study was conducted between 7 January 2020 and 6 February 2020. The studied population represents all 454 Slovenian primary schools in the school year 2019/2020. The participation of primary schools in the national cross-sectional study was on a voluntary basis and did not include any additional selection criteria. The schools that did not return the completed questionnaire were excluded from our study.

The chosen observation unit was the 3rd-grade classroom of each primary school. The questionnaires were sent via traditional mail to the headmasters of the primary schools. Along with the questionnaire, they received an invitation to participate and a consent form to participate, which they signed and returned in an enclosed envelope together with the completed questionnaire. The questionnaires were filled out by the 3rd-grade teachers of a primary school with the assistance of the caretaker.

2.2. Development and Content Validation of the Questionnaire

The questionnaire was developed based on a systematic literature review in the field of IAQ factors in educational settings and was content-validated [23,24] before its use. The questionnaire included the following three sections: (1) school building and school location information, (2) 3rd-grade classroom information and IAQ in the classroom, and (3) natural ventilation of the classroom. Content validation was performed among 6 experts (4 public health experts, 1 expert in the field of ventilation, and 1 in the field of school infrastructure) who were asked to give a score of either 0 (item not relevant) or 1 (item very relevant). Of the Content Validity Indices (CVIs), we calculated the scale content validity index (S-CVI/Ave), scale universal agreement validity index (S-CVI/UA), and a face item validity index (I-FVI). This was followed by response process validation among 12 raters who were asked to give a score of 0 or 1 based on the clarity and comprehensibility of the questionnaire. We checked their understanding of the whole questionnaire with them and the correctness of the answers. In accordance with their minor comments, we upgraded the questionnaire to make it fully understandable for the teachers. The questionnaire was used to collect data on the prevalence of indoor and outdoor IAQ factors in primary schools and teachers’ perceived IAQ for heating and non-heating seasons, based on their past experience [13]. In Slovenia, the heating season for primary schools includes the months from October to April, while the non-heating season is from May to September. The data on the heating season were obtained from the questionnaire from the Slovenian primary schools.

2.3. Data Analysis

The perceived IAQ was assessed using a five-point Likert scale. For further statistical analysis, the five-point Likert scale (5-very good, 4-good, 3-not good, not bad, 2-bad, 1-very bad) was modified to a three-point scale (3-good, 2-not good, not bad, 1-bad), where the ratings »1-very bad« and »2-bad« were combined into a rating »1-bad« and the ratings »4-good« and »5-very good« were combined into a rating »3-good«. The distribution of values is presented in a table form.

The statistical association between teachers’ perceived IAQ for the heating and non-heating seasons and selected IAQ factors (micro location, location of the school breakfast, potential sources of air pollution located within 200 m from the primary school, distance from the nearest major road, orientation of the classroom, flooring material, type of board and writing equipment, classroom cleaning method, manual airing during classroom cleaning, ventilation method) was tested using Pearsons’ Chi-Square or Fishers’ exact test. Responses of “Other” for selected IAQ factors were not included in the statistical analysis of their association. The statistical significance was defined at p ≤ 0.05. The statistical software SPSS version 21.0 (SPSS Inc., Chicago, IL, USA) was used to analyze the data.

3. Results

Out of 454 primary schools in Slovenia, 355 schools responded to the questionnaire, which corresponds to a response rate of 78.19%.

3.1. Association of Perceived Indoor Air Quality in the Classrooms and Micro Locations of Primary Schools

Teachers rated IAQ as the worst in winter and the best in autumn and spring. The average perceived IAQ rating for the heating season was 2.39 and 2.76 for the non-heating season. The results of the association between teachers’ perceived IAQ in the 3rd-grade classrooms and the micro location of Slovenian primary schools are shown in Table 1.

3.2. Association of Perceived Indoor Air Quality and Indoor Air Quality Factors in the Classrooms of Primary Schools

The results of the association between teachers’ perceived IAQ for the heating and non-heating seasons and selected IAQ factors are shown in Table 2.

4. Discussion

4.1. Association of Perceived Indoor Air Quality in the Classrooms and Micro Locations of Primary Schools

The results of our study show that teachers perceive classroom IAQ in Slovenian schools to be the worst in winter (58.33%), followed by summer (36.31%). In winter, classrooms are less ventilated via manual airing, which is most likely the reason for lower perceived IAQ ratings compared to other seasons. According to the study by Galičič et al. [13], during the heating season in Slovenia, 36.47% of classrooms are ventilated for less than 25 min per day, meaning that classrooms are not even manually aired between every break. The main role in manual airing of the classroom in our study was held by teachers (90.91%), which is consistent with the findings of Korsavi et al. [25], where the proportion of schoolchildren who participated in manual airing was comparable with our results. Further, the results of our study showed that classroom manual airing is mainly affected by the thermal comfort of occupations and outside noise. The classroom temperature has a significant effect on the perceived IAQ, which is better if the classroom temperature is under the individuals’ thermal comfort level [16]. The teachers have a higher comfort temperature compared to the schoolchildren, which means that they perceive poor IAQ later, and therefore, manual airing occurs later too [25]. Moreover, our results show that outside noise is statistically significantly associated with the micro location of the participating primary schools, with the greatest impact on reduced classroom manual airing in the city center (52.00%). Poor outdoor air quality should not be neglected, which was the reason for less frequent manual airing in a total of 8.85% of the participating schools, also with the greatest impact in the city center (10.00%). In addition to the lower manual airing frequency in winter compared to summer, manual airing for shorter periods during winter further impacts the classroom IAQ [26], when many researchers measured higher concentrations of pollutants in the classrooms [26,27,28]. Higher outdoor pollutant concentrations in winter have a significant impact on the occurrence of outdoor pollutants indoors [29].

According to the teachers, the classroom IAQ is better during the non-heating season than during the heating season. The classroom IAQ was rated as “good” and “very good” by 80.78% of the teachers for the non-heating season and by 50.15% for the heating season. The perceived classroom IAQ rated as “good” for the non-heating season is statistically significantly associated with the micro location of the participating primary schools. In the non-heating season, most teachers perceived the IAQ as “good” in the village/rural area (86.34%), followed by the suburbs/small town (78.86%) and the city center (66.00%). The prevalence of classroom IAQ rated as “good” for the non-heating season and also heating season is highest in the village/rural area and lowest in the city center, which suggests a possible association with the lower manual airing of classrooms in the city schools due to outdoor environmental factors. This is also shown in our results, in which the noise as a cause of lower classroom manual airing is statistically significantly higher in city center schools, which is more common in the cities due to a denser population and more traffic.

4.2. Association between Perceived Indoor Air Quality and Indoor Air Quality Factors in the Classrooms of Primary Schools

In our study, we conducted an analysis of teachers’ perceived IAQ in the 3rd-grade classrooms as an independent risk factor for public health and its association with IAQ factors, rather than focusing on its association with an individual air pollutant. The average perceived IAQ in the classroom for both heating and non-heating seasons was the highest for the village/rural area and the lowest for the city center. For the non-heating season, we found a statistically significant association between the perceived IAQ in the classroom and the micro location of the school in the city center and the village/rural area. For the non-heating season, there was also a statistically significant association between the perceived IAQ in the classroom and proximity (up to 200 m) to the industrial area and distance to the nearest major road, between 1 and 100 m. These two outdoor factors are expected to be more prevalent in the city center Slovenian schools [13] and have a greater impact on the classroom IAQ during the non-heating season when natural ventilation is more intense. Further, we found a statistically significant association between the perceived IAQ in the classroom for the heating season and the distance from the nearest major road within 201 to 500 m. Although, teachers in classrooms that were less (up to 200 m) or more distant (more than 501 m) from the nearest major road had worse perceived IAQ ratings. Most Slovenian schools are located less than 100 m away from a major road, with the highest prevalence of schools located in the city center (65.12%), followed by the village/rural area (56.25%) and the suburbs/small town (53.10%) [13]. Due to the lower distance from a major road and higher traffic density in urban areas, traffic has a greater impact on the IAQ in city center schools, while the impact of traffic only starts to decrease noticeably in schools 5 km or more away from the city [30].

The average perceived IAQ in the classroom for the non-heating season was the lowest for the classrooms oriented toward the traffic road, followed by the orientation toward a road with moderate traffic. A statistically significant association was found between perceived IAQ in the classroom and the orientation of the classroom towards the traffic road and the school playground for the non-heating season. Classrooms oriented towards the street are heavily affected by traffic pollutants [31,32], which may also be the reason for the lower perceived IAQ. For the heating season, our study shows a statistically significant association between the perceived IAQ in the classroom and the proximity (within 200 m) of a residential area with individual wood-burning stoves. Our finding is supported by Canha et al. [33], who found that residential areas with individual wood-burning stoves present an intensive source of emissions during the heating season.

The average rating of perceived IAQ is higher in classrooms where floors and surfaces were wet cleaned than in classrooms where dry cleaning was used. A statistically significant association was found between the perceived IAQ in the classroom for the heating season and the wet cleaning of floors and surfaces. A possible explanation for our result is the fact that wet cleaning, unlike dry cleaning, does not resuspend a larger amount of dust into the air, which would lead to an increased concentration of PM [34]. The statistically significant association between the perceived IAQ in the classroom for the heating season and manual airing during classroom cleaning cannot be explained.

The average rating of the perceived IAQ for the heating and non-heating season is higher in classrooms where flooring material was laminate and they used a plastic whiteboard and markers. The results of our study showed very small differences in the perceived IAQ according to different types of board and writing equipment.

4.3. Implications for School Health

The study provides important information that will contribute to a better understanding of the relevance of the perceived IAQ and its association with factors that affect both the perceived and actual degradation of the classroom IAQ. The long-term perception of poor IAQ in the classroom and the occurrence of associated symptoms is an important indicator of the potential exposure of occupants to elevated air pollutant concentrations, which should encourage school management to conduct appropriate IAQ measurements and address the issue. At the same time, the perceived IAQ is an important part of planning efficient natural ventilation of the classroom. Korsavi et al. [25] found that the IAQ in classrooms depends significantly on occupants’ good adaptive behaviors to manually air since the main ventilation method in most schools is natural ventilation. As mentioned before, the teachers hold the main role in the natural ventilation of the classroom and because they have a higher comfort temperature compared to schoolchildren, manual airing is carried out later than would be required [25]. In this regard, it is necessary to raise awareness among school management, teachers, and schoolchildren on the topic and educate them about good natural ventilation practices. This would result in a better IAQ in the classrooms as the frequency of natural ventilation would increase, particularly when occupants perceive the IAQ as poor. Schools with central mechanical ventilation and mechanical ventilation systems can provide continuous classroom ventilation and ventilation independent of occupants’ good manual airing habits [35]; however, compared to natural ventilation, a significant financial investment is required to set up these systems.

Further, this study, together with a previous study by Galičič et al. [13], provides critical relevant information on the different factors that may affect the IAQ in the classroom, which could support schools in addressing the most pressing issues in reducing the concentrations of indoor air pollutants.

4.4. Strengths and Limitations of the Study

The main limitation of our study is related to the perceived IAQ. The perceived IAQ could be biased because individual perception plays an important role in the perception of air quality. Nevertheless, air pollutants in the classroom have some obvious signs and influence on individual perception (e.g., elevated CO₂ concentrations cause fatigue and give a feeling of stuffy air, significantly increased PM concentrations lead to dustier surfaces in the classroom, and numerous VOCs also emit specific odors). It is important to consider that the perceived IAQ is also affected by the age of the individual assessing the perceived IAQ, as Järvi et al. [20] found that teachers were more likely to report problems with the IAQ and symptoms than schoolchildren. Another limitation of this study is that the questionnaire was used to collect data on the teachers’ evaluation of the perceived IAQ based on their past experience. However, this did not have an impact on our analysis of the association between the perceived IAQ and the IAQ factors, since these factors are always present and we did not investigate the intensity of each IAQ factor. Further, the data collected via the questionnaire refer to the period before the COVID-19 pandemic and were collected in the school year 2019/2020. During the COVID-19 pandemic in Slovenia, there was no investment in mechanical ventilation in the classrooms in educational settings, but the focus was on organizational measures, such as forming small learning groups, wearing a mask, and manual airing. However, our study data are still relevant, as current information from primary schools shows that the situation regarding the frequency of manual airing in classrooms has returned to the pre-COVID-19 period. Despite the limitations, our study provides important insights into teachers’ perceptions of the IAQ in Slovenian primary schools. Thus, this is the first study in Slovenia to associate the perceived IAQ and IAQ factors. A similar approach using perceived air quality has already been used in Slovenia to estimate the outdoor air quality [36,37]. Our study is distinguished by a high response rate of 78.19%, which meets the highest standards for questionnaire response rates [38]. Additionally, the selected population was large, representing all 454 primary schools in Slovenia in the 2019–2020 school year. An important advantage of the study is the use of a questionnaire that has been content-validated [13]. The study provides important information that can be used to develop guidelines to ensure good IAQ in the classroom and may also provide a starting platform for further research in this field.

4.5. Further Research

The results of our study showed that the majority of teachers perceive the IAQ as poor in the heating season, while about a fifth of teachers perceive it as poor in the non-heating season. Studies suggest that worse perceived IAQ in the classrooms is associated with higher concentrations and the presence of pollutants; therefore, in future research based on the results of our study, it would be reasonable to conduct detailed measurements of air pollutant levels in Slovenian primary school classrooms [16,17,19,21]. In addition, it would be useful to relate the results of these measurements to (1) the prevalence of IAQ factors in the monitored classrooms and (2) the IAQ perceived by teachers and schoolchildren. This would require the investigation of additional indoor factors that could affect the IAQ, such as the classroom activity of the users, the technical characteristics of the classroom/building, and a more detailed review of the materials and equipment. Furthermore, it would be relevant to investigate the effect of intense sunlight, solar radiation, high temperature, and the UV index on the perceived IAQ. The study could also be extended to kindergartens and secondary schools.

5. Conclusions

The most important finding of our study is that teachers perceive the IAQ in the classrooms to be the worst in the winter season. Only about half of the teachers perceive the IAQ in the classrooms as good during the heating season. The main reason for the less frequent natural ventilation of the classrooms during winter is the thermal discomfort of the occupants that would be caused by manual airing of the classroom. Further, the results of teachers’ perceived IAQ suggest that the micro location of the school is an important factor affecting the classroom IAQ. In our study, we recognized the need to raise awareness among classroom occupants about the importance of the IAQ and to improve their adaptive behavior towards more frequent natural ventilation of the classrooms to improve the IAQ in schools, as teachers have a major role in classroom manual airing.

Author Contributions

Conceptualization, A.G.; methodology, A.G., J.R. and A.K.; statistical analysis, A.G.; writing—original draft preparation, A.G. and J.R.; writing—review and editing, A.K.; supervision, I.E. and S.M. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

The research was approved by the Medical Ethics Committee of the Republic of Slovenia (No. 0120-548/2019/4).

Informed Consent Statement

Not applicable.

Data Availability Statement

Data supporting the findings of this study are available from the corresponding author upon reasonable request. The data are not publicly available due to privacy.

Acknowledgments

We would like to thank all headmasters of the participating primary schools for their high response to the request to participate in the study. We would like to thank the 3rd-grade teachers and the caretakers of the participating primary schools for their time and for accurately filling in the questionnaires. We would also like to thank all the colleagues and other members of the project “Development of the prognostic model of exposure to indoor air pollutants in schools and preparation of evidence based measures for planning of efficient natural ventilation of the classrooms (No. V3-1904)” who contributed to the development of our study.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Tables

Table 1

Association between teachers’ perceived indoor air quality in the 3rd-grade classrooms and the micro locations of Slovenian primary schools.

Variables	Description of Variables	Total: Number (Prevalence [%])	City Center:Number (Prevalence [%] for This Micro Location)	Suburbs/Small Town: (Prevalence [%] for This Micro Location)	Village/Rural Area: (Prevalence [%] for This Micro Location)	p
During which season of the year do you find the indoor air quality in your classroom to be the worst? (n = 336) *	Winter	196 (58.33)	32 (64.00)	67 (54.47)	97 (60.25)	0.429
	Spring	17 (5.06)	3 (6.00)	6 (4.88)	8 (4.97)	0.888
	Summer	122 (36.31)	22 (44.00)	45 (36.59)	55 (34.16)	0.572
	Autumn	13 (3.87)	1 (2.00)	7 (5.69)	5 (3.11)	0.498
How would you rate the indoor air quality in the classroom during the heating season? (n = 333)	Bad	35 (10.51)	6 (12.00)	13 (10.57)	16 (9.94)	0.893
	Not bad, not good	131 (39.34)	22 (44.00)	53 (43.09)	56 (34.78)	0.203
	Good	167 (50.15)	22 (44.00)	56 (45.53)	89 (55.28)	0.135
How would you rate the indoor air quality in the classroom during the non-heating season? (n = 333)	Bad	15 (4.50)	5 (10.00)	8 (6.50)	2 (1.24)	0.014
	Not bad, not good	49 (14.71)	12 (24.00)	17 (13.82)	20 (12.42)	0.106
	Good	269 (80.78)	33 (66.00)	97 (78.86)	139 (86.34)	0.004 **
What is the reason that you choose to manually air the classroom less often than you would otherwise? (n = 339) *	Outdoor air pollution	30 (8.85)	5 (10.00)	10 (8.13)	15 (9.32)	0.970
	Outdoor noise	98 (28.91)	26 (52.00)	36 (29.27)	36 (18.85)	0.001 **
	Thermal discomfort in the classroom (cold in winter, heat in summer)	228 (67.26)	33 (66.00)	88 (71.54)	107 (66.46)	0.542
	Draught	82 (24.19)	9 (18.00)	39 (31.71)	34 (21.12)	0.057
	Safety aspect	40 (11.80)	6 (12.00)	19 (15.45)	15 (9.32)	0.269
Who usually gives the initiative to manually air the classroom? (n = 319)	Teacher	290 (90.91)	44 (88.00)	108 (87.80)	138 (85.71)	0.521
	Schoolchildren	4 (1.25)	0 (0.00)	1 (0.81)	3 (1.86)	0.810
	Both teachers and schoolchildren	25 (7.84)	4 (8.00)	7 (5.69)	14 (8.70)	0.661

* Multiple choice question; ** the association is statistically significant (p ≤ 0.05).

Table 2

Association between teachers’ perceived indoor air quality in the 3rd-grade classrooms for the heating and non-heating seasons and selected indoor air quality factors.

Indoor Air Quality Factors in the Classrooms			Perceived Indoor Air Quality Vote for the Heating Season		Perceived Indoor Air Quality Vote for the Non-Heating Season
Variables	Description of Variables	Number	Average Rating	p	Average Rating	p
Primary school micro location (n = 334)	City center	49	2.32	0.640	2.56	0.011 **
	Suburbs/small town	124	2.35	0.467	2.73	0.383
	Village/rural area	161	2.45	0.303	2.85	0.010 **
Location of the school breakfast (n = 351)	In classroom	208	2.38	0.757	2.74	0.500
	In dining hall	143	2.43	0.773	2.79	0.621
Are there potential sources of air pollution located within 200 m from the primary school? (n = 350) *	Busy road	222	2.39	0.905	2.73	0.269
	Industrial area	18	2.42	0.620	2.63	0.032 **
	Individual industrial installations	38	2.51	0.133	2.76	0.450
	Residential areas with individual wood-burning stoves	201	2.34	0.018 **	2.75	0.574
	Other	56	/	/	/	/
Distance from the nearest major road (not the access road to the primary school) (n = 350)	0–100 m	169	2.38	0.566	2.69	0.006 **
	101–200 m	46	2.30	0.400	2.76	0.748
	201–500 m	60	2.60	0.027 **	2.90	0.063
	501–1000 m	18	2.28	0.133	2.78	0.092
	>1000 m	10	2.20	0.352	3.00	0.294
Orientation of the classroom (n = 353) *	Towards a traffic road	50	2.27	0.141	2.56	0.003 **
	Towards a road with moderate traffic	90	2.32	0.310	2.67	0.076
	Towards the school car park	55	2.37	0.089	2.82	0.312
	Towards school playground	99	2.47	0.328	2.82	0.018 **
	Towards school grounds park	85	2.44	0.572	2.79	0.795
	Other	37	/	/	/	/
Flooring material (n = 355) *	Parquet	151	2.37	0.710	2.76	0.799
	Laminate	6	2.67	0.611	3.00	0.484
	Synthetic materials (linoleum panels, vinyl panels, PVC, etc.)	197	2.42	0.882	2.77	0.971
	Other	2	/	/	/	/
Type of board and writing equipment (n = 346) *	Green chalkboard and chalk	282	2.37	0.194	2.75	0.672
	Plastic whiteboard and markers	169	2.40	0.752	2.77	0.680
	Interactive whiteboard and associated digital pen	142	2.35	0.489	2.74	0.706
Classroom cleaning method (n = 346) *	Wet cleaning of floors and surfaces	303	2.43	0.036 **	2.76	0.902
	Dry cleaning of floors and surfaces	72	2.27	0.130	2.77	0.679
Manual airing during classroom cleaning (n = 330)	Yes	180	2.49	0.026 **	2.80	0.128
	Not in winter, yes in summer	128	2.28	0.004 **	2.75	0.274
	No	22	2.50	0.725	2.64	0.480
Ventilation method (n = 322)	Natural ventilation	280	2.37	0.453	2.77	0.382
	Mechanical ventilation	13	2.35	0.974	2.76	0.415
	Hybrid ventilation	29	2.54	0.403	2.71	0.051

* Multiple choice question; ** the association is statistically significant (p ≤ 0.05).

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