Post-Wildfire Indoor Pollution in WUI Areas following the 2025 Los Angeles Fires. Part I. Establishing Baseline Contaminant Levels Prior to Home Reoccupation

Wildland-urban interface (WUI) fires pose unique environmental pollution challenges due to the combustion of both natural vegetation and synthetic building materials. Following the 2025 Palisades and Eaton wildfires in Los Angeles, we conducted a field study to characterize indoor air quality and surface contamination in 19 homes before reoccupancy. Indoor PM_2.5 and PM₁₀ concentrations averaged 3.45 and 31.66 μg/m³, respectively, with several homes showing indoor-to-outdoor (I/O) ratios of >1 (particularly for PM₁₀) compared to typical I/O values of 0.45-0.8 in residences, indicating persistent indoor particle reservoirs. Depending on the air-exchange rate, elevated indoor PM levels in noncleaned homes may arise from two contrasting mechanisms: low ventilation that traps resuspended fire residues triggered by movements during sampling and high ventilation that induces turbulence and disturbs heavily contaminated entry zones. Regression analysis suggested that proximity to the fire, absence of air purifiers, use of non-HEPA vacuums, and open windows during the fire significantly increased indoor PM levels, explaining 73% (PM₁₀) and 86% (PM_2.5) of the variation across homes. Airborne metal concentrations were below health-based thresholds; however, surface wipe samples revealed widespread contamination, with potassium, magnesium, aluminum, and iron frequently exceeding 1000 μg/ft², and detectable levels of zinc, copper, and manganese in many homes. Lead concentrations exceeded the EPA's dust clearance levels in multiple homes, especially on window sills and entry floors. Our findings highlight that while airborne risks may subside within weeks after the fire, indoor surfaces can retain fire-related pollutants, presenting ongoing exposure risks even 2 months after the fire.