Increased wildland fire activity is producing extreme fine particulate matter (PM2.5) concentrations impacting millions of people every year, especially in the western United States (US). Recommendations for limiting exposure to PM2.5 and associated adverse health outcomes focus on staying inside, closing windows and doors, and increasing filtration; however, relatively little is known about indoor air quality (IAQ) during major smoke events. Indoor and outdoor hourly PM2.5 (μg m -3 ) measurements from the publicly available PurpleAir sensor (PAS) network 19 were analyzed for 42 sites (26 residential, 6 school, 10 commercial) across the western US during 20 a September 2020 period of heavy wildfire smoke influence. The fraction of ambient PM2.5 that penetrates indoors and remains airborne (Fin), as well as the ratio (I/O) and correlation coefficient 22 (R 2 ) of indoor to outdoor PM2.5 concentrations, were lower in residential compared to commercial 23 and school buildings. Interventions to improve IAQ were highly influential in PM2.5 infiltration in 24 residential case studies, with multiple, continuously run filter units associated with lower Fin, I/O, 25 and R 2 . A low-cost PM2.5 filtration method consisting of a Minimum Efficiency Rating Value-13 (MERV-13) filter attached to a box fan is evaluated as an alternative for improving IAQ during wildland fire smoke events. The MERV-13 fan filter unit proved highly effective at reducing indoor PM2.5 and particles 0.3 -1.0 μm measured by PAS and a particle counter, respectively, 29 when recirculating air in a single room. Low-cost filtration methods can have significant benefit for filtering submicron smoke particles and may reduce exposure to PM2.5 during wildfire smoke events.