Number of school buildings investigated: 16
Monitoring campaigns were conducted during the heating season of 2017/2018.

HU01 is located in the city centre of Várpalota, close to a relatively high traffic road. The school building was built from brick at the beginning of the 18th century. The indoor air quality was in the unhealthy category based on the indoor health index. The main air pollutants were benzene and particulate matter (PM2.5). It should be noted that the outdoor value for the PM2.5 and benzene mass concentrations were also high, thus the inappropriate indoor air quality was mainly caused by the outdoor air pollution. Furthermore, one of the comfort parameters was also in the unhealthy range; the relative humidity was low in the classroom and the carbon dioxide concentration was in the moderate range.

HU04 is a provincial school in Péteri, close to a relatively high traffic road. The school building was built from brick at the end of the 20th century. The indoor air quality was in the unhealthy category based on the indoor health index. The main air pollutant was particulate matter (PM2.5). It should be noted that the outdoor value for the PM2.5 mass concentration was also high, thus the inappropriate indoor air quality was mainly caused by the outdoor air pollution. Furthermore, one of the comfort parameters was in the moderate range; the relative humidity was low in the classroom.

HU05 is located in the residential area of Várpalota, far away from high traffic roads. The school building was built from brick in the middle of the 20th century. The indoor air quality was in the moderate category based on the indoor health index. The main air pollutants were benzene and particulate matter (PM2.5). It should be noted that the outdoor value for the PM2.5 and benzene mass concentrations were also high, thus the inappropriate indoor air quality was mainly caused by the outdoor air pollution. Furthermore, one of the comfort parameters was also in the moderate range and the carbon dioxide concentration was relatively high in the classroom.

HU06 is located in an outer district of Budapest, close to a busy intersection. The school building was built from brick in the middle of the 20th century. The indoor air quality was in the unhealthy category based on the indoor health index. The main air pollutants were benzene and particulate matter (PM2.5). It should be noted that the outdoor values for the PM2.5 mass concentration and benzene concentration were also high, thus the inappropriate indoor air quality was mainly caused by the outdoor air pollution. Furthermore, one of the comfort parameters was also in the unhealthy range; the relative humidity was low in the classroom.

HU07 is located in an outer district of Budapest, far away from high traffic roads. The school building was built from concrete at the end of the 20th century. The indoor air quality was in the very unhealthy category based on the indoor health index. The main air pollutant was particulate matter (PM2.5). It should be noted that the outdoor value for the PM2.5 mass concentration was considerably low, thus the inappropriate indoor air quality was mainly caused by the indoor air pollutant source. Furthermore, one of the comfort parameters was in the unhealthy range; the relative humidity was low in the classroom.

HU09 is located in the residential area of Budapest, close to a road characterized by moderate traffic density. The school building was built from concrete, at the end of the 20th century. The indoor air quality was in the unhealthy category based on the indoor health index. The main air pollutant was particulate matter (PM2.5). It should be noted that the outdoor value for the PM2.5 mass concentration was also high, thus the inappropriate indoor air quality was mainly caused by the outdoor air pollution. Furthermore, one of the comfort parameters was in the moderate range; the relative humidity was low in the classroom.

HU15 is located in the city centre of Várpalota, close to a relatively high traffic road. The school building was built from brick in the middle of the 20th century. The indoor air quality was in the very unhealthy category based on the indoor health index. The main air pollutant was particulate matter (PM2.5). It should be noted that the outdoor value for the PM2.5 mass concentration was considerably low, thus the inappropriate indoor air quality was mainly caused by the indoor air pollutant source. Furthermore, one of the comfort parameters was in the unhealthy range; the relative humidity was low in the classroom and the carbon dioxide concentration was in the moderate range.

HU17 is located in the residential area of Budapest, close to a relatively high traffic road. The school building was built from brick, at the beginning of the 20th century. The indoor air quality was in the unhealthy category based on the indoor health index. The main air pollutant was particulate matter (PM2.5). It should be noted that the outdoor value for the PM2.5 mass concentration was also high, thus the inappropriate indoor air quality was caused mainly by the outdoor air pollution. Furthermore, one of the comfort parameters was in the unhealthy range; the relative humidity was low in the classroom.

HU19 is located in the residential area of Budapest, close to a road characterized by moderate traffic density. The school building was built from brick at the end of the 20th century. The indoor air quality was in the unhealthy category based on the indoor health index. The main air pollutant was particulate matter (PM2.5). It should be noted that the outdoor value for the PM2.5 mass concentration was also high, thus the inappropriate indoor air quality was mainly caused by the outdoor air pollution. Furthermore, one of the comfort parameters was in the moderate range; the carbon dioxide concentration was relatively high in the classroom.

HU20 is located in the residential area of Szentendre, close to a relatively high traffic road. The school building was built from concrete at the end of the 20th century. The indoor air quality was in the very unhealthy category based on the indoor health index. The main air pollutant was particulate matter (PM2.5). It should be noted that the outdoor value for the PM2.5 mass concentration was also high, thus the inappropriate indoor air quality was mainly caused by the outdoor air pollution. Furthermore, two of the comfort parameters were in the moderate range; the relative humidity was low and the carbon dioxide was relatively high in the classroom.

HU21 is located in the city centre of Karcag, close to a relatively high traffic road. The school building was built from brick, in the middle of the 20th century. The indoor air quality was in the very unhealthy category based on the indoor health index. The main air pollutant was particulate matter (PM2.5). It should be noted that the outdoor value for the PM2.5 mass concentration was considerably low, thus the inappropriate indoor air quality was mainly caused by the indoor air pollutant source. Furthermore, one of the comfort parameters was in the unhealthy range; the carbon dioxide concentration was relatively high in the classroom.

HU22 is located in the city centre of Budapest, close to a relatively high traffic road. The school building was built from brick at the end of the 19th century. The indoor air quality was in the very unhealthy category based on the indoor health index. The main air pollutants were benzene and particulate matter (PM2.5). It should be noted that the outdoor values for the concentration benzene and PM2.5 mass were also high, thus the inappropriate indoor air quality was mainly caused by the outdoor air pollution. Furthermore, one of the comfort parameters was in the unhealthy range; the relative humidity was low in the classroom.

HU23 is located in the city centre of Budapest, close to a relatively high traffic area. The school building was built from brick, at the beginning of the 20th century. The indoor air quality was in the unhealthy category based on the indoor health index. The main air pollutants were benzene and particulate matter (PM2.5). It should be noted that the outdoor value for the PM2.5 mass concentration was higher than indoor value, thus the inappropriate indoor air quality was caused mainly by the outdoor air pollution. Furthermore, one of the comfort parameters was also in the unhealthy range; the relative humidity was low in the classroom.

HU24 is located in the residential area of Budapest, close to a road characterized by moderate traffic density. The school building was built from metal at the end of the 20th century. The indoor air quality was in the unhealthy category based on the indoor health index. The main air pollutants were benzene and particulate matter (PM2.5). It should be noted that the outdoor value for the PM2.5 mass concentration was also high, thus the inappropriate indoor air quality was mainly caused by the outdoor air pollution. Furthermore, one of the comfort parameters was also in the unhealthy range; the relative humidity was low in the classroom.

HU25 is located in the residential area of Budapest, close to a relatively high traffic road. The school building was built from brick, at the beginning of the 20th century. The indoor air quality was in the unhealthy category based on the indoor health index. The main air pollutant was particulate matter (PM2.5). It should be noted that the outdoor value for the PM2.5 mass concentration was also high, thus the inappropriate indoor air quality was mainly caused by the outdoor air pollution. Furthermore, one of the comfort parameters was in the moderate range; the relative humidity was low in the classroom.

HU26 is located in Debrecen, close to a relatively high traffic road. The school building was built from brick in the middle of the 20th century. The indoor air quality was in the very unhealthy category based on the indoor health index. The main air pollutant was particulate matter (PM2.5). It should be noted that the outdoor value for the PM2.5 mass concentration was high, thus the inappropriate indoor air quality was mainly caused by the outdoor air pollution. Furthermore, one of the comfort parameters was in the unhealthy range; the carbon dioxide concentration was relatively high in the classroom.