2019 No https://www.sciencedirect.com/science/article/abs/pii/S0013935119305882?via%3Dihub Kalimeri, Krysallia, K. Europe Observational Exposure Study Elementary (K-5) Ozone (O3) Nitrogen Dioxide (NO2) Fine particles Nitrogen dioxide (NO2) Ozone (O3) Offices School The PM2.5 I/O ratio varied from 0.20 to 9.29, with mean PM2.5 I/O ratio equal to 1.44 ± 1.01 (mean ± σ value). In schools only 41% of the I/O ratios are equal or below unity, in contrast with the offices present data (97%). PM2.5 I/O ratios above unity are common for school environments.