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Abstract
Wildfires have become the dominant 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 over 1400 buildings from the crowdsourced PurpleAir sensor network in California, we found that infiltration ratios (indoor PM2.5 of outdoor origin/outdoor PM2.5) were reduced on average from 0.4 during non-fire days to 0.2 during wildfire days. Even with reduced infiltration, mean indoor concentration of PM2.5 nearly tripled during wildfire events, with lower infiltration in newer buildings and those utilizing air conditioning or filtration.
