2019 Yes https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6360122/ Majd, Ehsan North America Observational Epidemiology Study Elementary (K-5) Middle/Jr High (6-8) Ventilation rates Filtration Temperature Humidity Carbon Dioxide (CO2) Carbon Monoxide (CO) Nitrogen Dioxide (NO2) Indoor air quality School Exposure Building characteristics Environmental factors Exceedance of the WHO guidelines for indoor air was observed, mainly for the hourly average NO2 concentrations. Seasonal variability was statistically significant for indoor NO2 and CO concentrations, with higher exposures in fall and winter. Factors with significant contributions to indoor exposure were mostly related to the outdoor pollution sources. Additionally, we found that poor conditions of the buildings (a prevalent phenomenon in the studied urban area), including physical defects and lack of proper ventilation, contributed to poor air quality in schools. The results suggest that improving building conditions and facilities as well as a consideration of the school surroundings may improve indoor air quality in schools.