Socioeconomic inequity of measured indoor and outdoor exposure to PM2.5: 5 Years of data from 14,000 low-cost particle monitors

Multiple studies have considered socioeconomic or ethnic group inequities in outdoor fine particle (PM_2.5) concentrations. Due to the lack of indoor measurements, these studies are forced to assume that indoor exposures are directly related to outdoor concentrations. In general, this assumption may be reasonable, but it is violated when indoor-generated fine particles form a substantial contribution to total potential indoor exposure. We now have for the first time access to long-term (months or years) indoor potential exposures, made possible by the development of low-cost optical particle counters. A large database of 120 million hourly PM_2.5 indoor and outdoor concentrations measured by 10,000 outdoor and >4000 indoor PurpleAir monitors over a 5-year period (2017–2021) in three West Coast states (Washington, Oregon, California) has been used to compare with US Census 2021 estimates of median household income, educational attainment, housing characteristics, and ethnic groups. Clear evidence of inequities is found using indoor as well as outdoor PM_2.5 concentrations.