Indoor PM2.5 concentrations in China: A concise review of the literature published in the past 40 years

Fine particulate matter (PM2.5) pollution is of national concern in China. Indoor PM2.5 concentrations may contribute significantly to human exposure because people spend most of their time indoors. In this study, we systematically reviewed the level of indoor PM2.5 concentrations in residences, offices, and schools in China. We looked for corresponding literature published between 1980 and 2019 in six major literature databases and finally obtained 118 articles. We then summarized the spatial distribution of indoor PM2.5 concentrations in urban and rural areas, analyzed the indoor PM2.5 concentrations in residences, offices, and schools in urban areas across three time periods, and compared the indoor PM2.5 concentration levels in the urban and rural areas. It was found that the urban areas considered are mainly distributed in the middle and eastern part of China, while the rural areas are mainly distributed in northeast, north, and southwest China. Indoor PM2.5 concentrations have decreased significantly in the past decades. The mean ± SD value of indoor PM2.5 concentration decreased from 127.5 ± 111.4 μg/m³ prior to 2010 to 70.1 ± 35.2 μg/m³ following 2015. The indoor PM2.5 concentration in rural areas was much higher than that in urban areas. The mean ± SD value of indoor PM2.5 concentrations in rural residences was up to 275.7 ± 326.4 μg/m³, while that in urban residences was 95.4 ± 71.0 μg/m³. A long-term national indoor PM2.5 concentration measurement is suggested to obtain a more comprehensive and accurate indoor PM2.5 concentration level.