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/m 3 ) and indoor levels 91% (10 to 19 µg/m 3 ) during the fire. HEPA homes had indoor PM 2.5 levels 3 µg/m³ (15%, p = 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.