China has experienced serious ambient air quality deterioration.One promising approach to reduce air pollution exposure is to use portable air cleaners in indoor environments,the effectiveness of which has not been reported in high air pollution areas(e.g.,urban areas in China)for multiple air pollutants(e.g.primary and secondary pollutants).This air pollution intervention study aims to test the effectiveness of a portable air cleaner on exposure and risk reduction in residential homes in China under real-world exposure conditions.A portable air cleaner(Philips AC 4374 and Philips multilayer HEPA Filter 4138)was applied in twenty residential homes in Chongqing(southwest China)from summer,2015 to winter,2016.Each home was followed for four consecutive days: the air cleaner was operated with the HEPA filter installed for two days and without the filter installed for the other two days.Simultaneous indoor and outdoor measurements were conducted for PM2.5,black carbon,NO2,O3,formaldehyde,OC/EC,temperature and humidity,and CO2.The health benefits associated with PM2.5 exposure reduction were estimated using concentration-response curvesfrom the Global Burden of Disease project.The portable air cleaner significantly(p<0.0001,mixed effect models)reduced indoor PM2.5 by 19 μg/m3 or 23%,1.8 μg/m3 or 32%for BC,1.4 ppb or 11%for O3,and 26 μg/m3 or 26%for NO2; and reduced the infiltration of outdoor PM2.5 by 42%,26%for O3,17%for NO2,and 12%for BC.This exposure mitigation reduced the population attributable risk for PM2.5 from 18%to 13%for chronic obstructive pulmonary disease(COPD)and from 24%to 17%for lung cancer.A portable air cleaner significantly reduced indoor air pollutants in a high air pollution city.The reduction in personal exposures to PM2.5 leads to a reduction in population attributable risks associated with COPD and lung cancer mortality.