Impact of Wildfire Smoke Events on Indoor Air Quality and Evaluation of a Low-cost Filtration Method

Abstract

Increased wildland fire activity is producing extreme fine particulate matter (PM_2.5) concentrations impacting millions of people every year, especially in the western United States (US). Recommendations for limiting exposure to PM_2.5 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 PM_2.5 (µg m^–3) measurements from the publicly available PurpleAir sensor (PAS) network were analyzed for 42 sites (26 residential, 6 school, 10 commercial) across the western US during a September 2020 period of heavy wildfire smoke influence. The fraction of ambient PM_2.5 that penetrates indoors and remains airborne (F_in), as well as the ratio (I/O) and correlation coefficient (R²) of indoor to outdoor PM_2.5 concentrations, were lower in residential compared to commercial and school buildings. Interventions to improve IAQ were highly influential in PM_2.5 infiltration in residential case studies, with multiple, continuously run filter units associated with lower F_in, I/O, and R². A low-cost PM_2.5 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 PM_2.5 and particles 0.3–1.0 µm measured by PAS and a particle counter, respectively, 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 PM_2.5 during wildfire smoke events.