2020 Yes https://onlinelibrary.wiley.com/doi/abs/10.1111/ina.12771 Tham, Kwok Wai Asia Exposure Study Filtration In this study, it was observed that the average ambient PM10, PM2.5 and PM1 concentrations were 110, 85 and 70 μg m−3 respectively. Acceptable limits during haze episodes may lead to potential health hazards to occupants.43 The majority of classrooms are naturally ventilated, and the PM levels during haze episodes exceed threshold acceptable values.