Indoor sources strongly contribute to exposure of Chinese urban residents to PM2.5 and NO2.

Ambient fine particulate matter (diameters <2.5 µm; PM2.5) and nitrogen dioxide (NO2) pollution are responsible for substantial health burdens in China, contributing to a considerable proportion of global mortality. Simultaneously, the proportion of indoor smoking and cooking-induced PM2.5 and NO2 pollution lacks robust exposure assessment findings. Rapid poverty alleviation and urbanization affect the proportion of indoor vs outdoor sources of PM2.5 and NO2 exposures in China. The current understanding of air pollution and health lacks an understanding of source-specific air pollution exposure. Thus, we developed a model to estimate human exposure to pollutants originating indoors and outdoors. We found indoor sources strongly contribute to total PM2.5 and NO2 exposure in urban China and are comparable to outdoor sources. Cooking contributes 28.6 μg m-3 PM2.5 and 10.8 μg m-3 NO2 on average to the air people breathe, and so did smoking contributing 14.2 μg m-3 PM2.5 and 0.6 μg m-3 NO2, respectively. The results give us a clearer understanding of exposure to PM2.5 and NO2 from indoor and outdoor sources. Pollutant control policies on ambient exposure levels without addressing indoor air pollution in China are insufficient given our estimated exposure levels.