Wildfire Smoke Impacts on Indoor Air Quality Assessed Using Crowdsourced Data in California

Wildfires have become the dominant source of particulate matter (PM_2.5, < 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 PM_2.5 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 PM­_2.5 of outdoor origin/outdoor PM_2.5) 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 PM_2.5 nearly tripled during wildfire events, with lower infiltration in newer buildings and those utilizing air conditioning or filtration.