Significance Wildfires are an increasingly large source of particulate matter (PM _2.5 ) in the western United States. Previous characterizations of exposure to wildfire smoke particles were based mainly on outdoor concentrations of PM _2.5 . Since people mainly shelter indoors during smoke events, the infiltration of wildfire PM _2.5 into buildings determines exposure. We present analysis of infiltration of wildfire PM _2.5 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 PM _2.5 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.