Wildfire smoke impacts on indoor air quality assessed using crowdsourced data in California

Significance Wildfires are an increasingly large source of particulate matter (PM2.5) in the western United States. Previous characterizations of exposure to wildfire smoke particles were based mainly on outdoor concentrations of PM2.5. Since people mainly shelter indoors during smoke events, the infiltration of wildfire PM2.5 into buildings determines exposure. We present analysis of infiltration of wildfire PM2.5 into more than 1,400 buildings in California using more than 2.4 million sensor hours of data from the PurpleAir sensor network. Our study reveals that infiltration of PM2.5 during wildfire days was substantially reduced compared with non-fire days, due to people’s behavioral changes. These results improve understanding of exposure to wildfire particles and facilitate informing the public about effective ways to reduce their exposure. Wildfires have become an important source of particulate matter (PM2.5 < 2.5-µm diameter), leading to unhealthy air quality index occurrences in the western United States. Since people mainly shelter indoors during wildfire smoke events, the infiltration of wildfire PM2.5 into indoor environments is a key determinant of human exposure and is potentially controllable with appropriate awareness, infrastructure investment, and public education. Using time-resolved observations outside and inside more than 1,400 buildings from the crowdsourced PurpleAir sensor network in California, we found that the geometric mean infiltration ratios (indoor PM2.5 of outdoor origin/outdoor PM2.5) were reduced from 0.4 during non-fire days to 0.2 during wildfire days. Even with reduced infiltration, the mean indoor concentration of PM2.5 nearly tripled during wildfire events, with a lower infiltration in newer buildings and those utilizing air conditioning or filtration.