Residents of polluted cities frequently use indoor air purifiers in an attempt to improve their health by reducing their exposure to air pollutants, although there have been few studies assessing the filtration devices under relevant field settings. Low-cost air monitors are becoming increasingly popular for monitoring air pollution exposure, however they must first be calibrated and evaluated in their deployment location to ensure measurement accuracy and precision. In this study, we present a 2-step calibration method in which a low-cost monitor is calibrated against a reference analyzer and is then used to calibrate other monitors shortening the required calibration time and reducing measurement error. These low-cost monitors measured indoor, outdoor and personal exposure PM2.5 concentrations during 1 week each of true and sham filtration in 7 homes in Beijing, China. Filtration reduced indoor and personal exposure relevant concentrations by on average 72% (std. err. = 7%) and 28% percent (std. err. = 5%), respectively. This study highlights that to achieve even lower personal exposure, additional actions must be taken to reduce outdoor air infiltrating the home, or actions need to be taken at the city or country level to reduce PM2.5 pollution.