Indoor and Outdoor Exposure to LA Urban Wildfire Smoke – A Nature Experiment between the Palisades and Eaton Fires - ABSTRACT The Los Angeles urban wildfires (LA Fires), which began on January 7, 2025, have become one of the most destructive disasters in U.S. history. Although officially contained by February 7, 2025, emerging evidence sug- gests that exposure risks persist long after active burning due to post-fire degassing and cleanup activities. LA Fires released a complex mix of volatile and semi-volatile organic compounds (VOCs and SVOCs) from burned structures, vehicles, and household products. These emissions can last weeks to months, as damaged materials release pollutants back into the air. Cleanup activities, including debris removal and demolition, may further re- suspend contaminated dust and release VOCs, prolonging exposure risks for returning residents. However, post- fire indoor air quality remains an understudied yet critical issue in wildfire research. Since January 8, 2025, our team has monitored indoor and outdoor air quality in 25 households near both fires. We found outdoor concen- trations of benzene, toluene, ethylbenzene, xylenes (BTEX), and toxic heavy metals were over 100 times higher during active burning. In the post-fire phase, indoor BTEX levels in burn zone homes exceeded outdoor concen- trations, even in the absence of indoor activities, suggesting prolonged off-gassing from building materials. Ad- ditionally, real-time monitoring detected persistently elevated ultrafine and sub-micron particles in the burn zones weeks after the fires were contained. These findings underscore the urgent need to characterize post-fire expo- sure pathways, particularly in indoor environments where people spend most of their time.To address these critical gaps, we propose two aims. Aim 1: Assess indoor and outdoor exposure to fine and ultrafine particles and VOCs post-fire. We will expand sampling to 50 homes (25 per fire) and monitor air quality over one year or until cleanup is complete. Homes will include affluent, predominantly White communities (Palisades) and pre- dominantly Black communities (Altadena). We hypothesize that indoor VOC levels will remain elevated due to prolonged off-gassing, while fine and ultrafine particles will persist outdoors due to dust resuspension and sec- ondary aerosol formation. Aim 2: Evaluate how building characteristics and mitigation measures influence indoor air quality. We hypothesize that older, leakier homes will experience greater infiltration of outdoor fire smoke, leading to higher post-fire indoor VOC levels. We further hypothesize that wealthier residents in the Palisades will implement more effective mitigation measures (e.g., HVAC upgrades, air purifiers) compared to residents in Altadena, leading to lower indoor exposures. With over 16,000 structures destroyed, the LA Fires released highly toxic pollutants, posing persistent exposure risks to affected communities. This study will provide time-sensitive data to guide returning residents, inform evidence-based policies, and improve indoor air quality recommenda- tions for filtration and ventilation. Findings will also lay the groundwork for future research on the long-term health impacts of toxic urban wildfire smoke, a growing concern as wildfires increasingly threaten densely populated areas.
