Fine particulate matter levels and HEPA filtration in Los Angeles homes during a wildland-urban-interface fire

Wildland and wildland-urban-interface (WUI) fires substantially elevate fine particulate matter (PM_2.5) concentrations in surrounding communities. Portable high-efficiency particulate air (HEPA) purifiers are widely recommended to reduce indoor PM_2.5 exposure during such events, yet this guidance largely derives from studies of traffic exhaust, secondhand smoke, or indoor sources, rather than real-world WUI fire episodes. To address this gap, we leveraged data from ongoing randomized crossover trials of long-term use of portable HEPA purifiers in Los Angeles residences. During the Eaton Fire (January 2025), 11 homes were under HEPA intervention and 16 under non-HEPA control. Continuous indoor and outdoor PM_2.5 monitoring before, during, and after the Eaton Fire (over 6 weeks) showed outdoor PM_2.5 levels rose 148% (19 to 47 µg/m³) and indoor levels 91% (10 to 19 µg/m³) during the fire. HEPA homes had indoor PM_2.5 levels 3 µg/m³ (15%, <i>p</i> = 0.01) lower than non-HEPA homes, while outdoor concentrations were comparable. Indoor PM_2.5 reductions were unaffected by pre-filter use or clean air delivery rate (CADR)-to-room-volume ratio (0.8-4.9). These findings indicate that portable HEPA purifiers provided statistically significant but modest reductions in PM_2.5 levels during WUI fire events. Complementary building-level and behavioral interventions remain critical to reduce indoor exposure in fire-affected communities.