Indoor contribution to PM_{2 .5} exposure using all PurpleAir sites in Washington, Oregon, and California

Low-cost monitors have made it possible for the first time to measure indoor PM_2.5 concentrations over extended periods of time (months to years). Coupled with concurrent outdoor measurements, these indoor measurements can be divided into particles entering the building from outdoors and particles generated from indoor activities. Indoor-generated particles are not normally considered in epidemiological studies, but they can have health effects (e.g., passive smoking and high-temperature cooking). We employed The Random Component Superposition (RCS) regression model to estimate infiltration factors for up to 790 000 matched indoor and outdoor sites. The median infiltration factors for subgroups in the 3-state region ranged between 0.22 and 0.24, with an interquartile range (IQR) of 0.13-0.40. These infiltration factors allowed calculation of both the indoor-generated and outdoor-infiltrated PM_2.5 . Indoor-generated particles contributed, on average, 46%-52% of total indoor PM_2.5 concentrations. However, the site-specific fractional contribution of these indoor sources to total indoor PM_2.5 ranged from near-zero to nearly 100%. The influence of indoor-generated particles on potential exposures varied widely relative to outdoor concentrations. The greatest influence of indoor-generated particles occurred at low-to-moderate daily mean outdoor PM_2.5 levels around 6 μg/m³ and was negligible at outdoor concentrations >20 μg/m³ . Epidemiological studies incorporating only estimated exposures due to the particles of ambient origin may benefit from the newly available knowledge of long-term indoor-generated particle concentrations.