The impact of air filtration units on primary school students' indoor exposure to particulate matter in China.

The air quality in classrooms is a critical factor that affects students' daily exposure. Although air filtration units (AFUs) are often used to reduce exposure to air pollution in China, the effectiveness of installing AFUs in Chinese classrooms have not been quantitatively studied. Hence, we carried out concurrent air quality experiments in two identical classrooms where one room was equipped with commercial AFUs and the other one relied on the natural introduction of outside air. Measurements were taken during regular school days in the winter at a primary school in Hangzhou, China. Three AFU ventilation modes, i.e., fresh air, mixed air and recirculation mode were evaluated, respectively. We found that classrooms equipped with AFU showed significant PM reductions, but this was accompanied by CO build-ups. In classroom with AFUs on internal recirculation, nearly 70% of the PM concentration at the beginning of the class was reduced by the end of a 40-min class. However, the CO concentration was observed to reach levels that were six times greater than that of the ambient due to inadequate air change rate (<1 h). To mitigate the issue of excessive in-classroom CO accumulation when using AFUs, filtered outside air must be brought in during their operation. In comparison with the internal recirculation mode, we demonstrated that the fresh air mode that draws the supply air from outdoor can considerably lower CO build-ups in the classroom with a moderate increase in PM concentration of 15 ± 10 μg/m. The findings from this study helps policy makers especially in developing countries with serious air pollution issues to determine whether or not to install AFU in primary schools and what ventilation mode is effective in reducing negative health effects.