Characterizing outdoor infiltration and indoor contribution of PM2.5 with citizen-based low-cost monitoring data

Epidemiological research on the adverse health outcomes due to PM₂.₅ exposure frequently relies on measurements from regulatory air quality monitors to provide ambient exposure estimates, whereas personal PM₂.₅ exposure may deviate from ambient concentrations due to outdoor infiltration and contributions from indoor sources. Research in quantifying infiltration factors (Fᵢₙf), the fraction of outdoor PM₂.₅ that infiltrates indoors, has been historically limited in space and time due to the high costs of monitor deployment and maintenance. Recently, the growth of openly accessible, citizen-based PM₂.₅ measurements provides an unprecedented opportunity to characterize Fᵢₙf at large spatiotemporal scales. In this analysis, 91 consumer-grade PurpleAir indoor/outdoor monitor pairs were identified in California (41 residential houses and 50 public/commercial buildings) during a 20-month period with around 650000 h of paired PM₂.₅ measurements. An empirical method was developed based on local polynomial regression to estimate site-specific Fᵢₙf. The estimated site-specific Fᵢₙf had a mean of 0.26 (25ᵗʰ, 75ᵗʰ percentiles: [0.15, 0.34]) with a mean bootstrap standard deviation of 0.04. The Fᵢₙf estimates were toward the lower end of those reported previously. A threshold of ambient PM₂.₅ concentration, approximately 30 μg/m³, below which indoor sources contributed substantially to personal exposures, was also identified. The quantified relationship between indoor source contributions and ambient PM₂.₅ concentrations could serve as a metric of exposure errors when using outdoor monitors as an exposure proxy (without considering indoor-generated PM₂.₅), which may be of interest to epidemiological research. The proposed method can be generalized to larger geographical areas to better quantify PM₂.₅ outdoor infiltration and personal exposure.