2019 Yes https://www.sciencedirect.com/science/article/pii/S0160412018324140 Carrion-Matta, Aleshka North America Observational Classroom indoor concentrations of PM 2.5 (an average of 5.2 μg/m3) were lower than outdoors (an average of 6.5 μg/m3), and these averages were in the lower range compared to the findings in other schools' studies. The USEPA PMF model was applied to the PM2.5 components measured simultaneously from classroom indoor and outdoor to estimate the source apportionment. The major sources (contributions) identified across all seasons of indoor PM 2.5 were secondary pollution (41%) and motor vehicles (17%), followed by Calcium (Ca)-rich particles (12%), biomass burning (15%), soil dust (6%), and marine aerosols (4%). Likewise, the major sources of outdoor PM 2.5 across all seasons were secondary pollution (41%) and motor vehicles (26%), followed by biomass burning (17%), soil dust (7%), road dust (3%), and marine aerosols (1%). Secondary pollution was the greatest contributor to indoor and outdoor PM 2.5 over all three seasons, with the highest contribution during spring with 53% to indoor PM 2.5 and 45% to outdoor PM 2.5.