A two-year indoor–outdoor monitoring study of household clean air interventions during ambient and wildfire smoke periods in King County, Washington

Abstract Wildfire smoke (WFS) season is lengthening and intensifying with climate change, creating the need for effective adaptation strategies for reducing exposure to poor air quality during smoke events. To better understand the utility of low-cost air sensors and portable air filtration devices as household-level interventions for improving indoor air quality (IAQ) during WFS events, we launched a participatory science study among select King County, Washington households during summer 2023 and 2024, coinciding with wildfire season. Participants received indoor PurpleAir sensors, do-it-yourself (DIY) box fan filter kits, high efficiency particulate air (HEPA) portable air cleaners, and surveys on device use. We compared hourly fine particulate matter (PM 2.5 ) concentrations from the indoor sensors to those from proximate outdoor PurpleAir sensors. We ran linear mixed-effects models to assess the influence of WFS and building characteristics on IAQ and to evaluate the efficacy of air filtration use (2024 only). Twelve households in 2023 and 24 households in 2024 completed the study. In 2023, the mean indoor and outdoor PM 2.5 concentrations were higher than those in 2024 (a difference of 2.62 µ g m −3 and 1.74 µ g m −3 , respectively), reflecting different trends in air quality and WFS events. In 2024, both filtration devices were associated with reductions in indoor PM 2.5 concentrations; however, effect estimates differed after accounting for outdoor PM 2.5 , with HEPA portable air cleaners associated with a greater reduction than DIY box fan filters. The occurrence of a WFS event, warmer outdoor temperature, and higher appraised household value were significantly associated with higher indoor PM 2.5 concentrations, although a large portion of the model’s variance remained unexplained. These findings suggest that low-cost air sensors and air filtration devices may be effective IAQ interventions and encourage future investigations with larger sample sizes and more precise measures of air filtration use.