Health risks from smoke damage may linger for residents who returned to their homes after the 2025 Los Angeles County wildfires were extinguished, according to new research led by UCLA Fielding School of Public Health, published Friday.
Researchers found that, compared with the period when a fire was burning, the levels of some volatile organic compounds were significantly higher in the post-fire period and were particularly evident in uninhabited homes within burn zones. The finding suggests that unhealthy indoor emissions of fire-related pollutants continue even after a fire is extinguished.
“These findings underscore the need for targeted interventions to minimize indoor exposures during the recovery phase,” study co-author Dr. Michael Jerrett, a professor in UCLA Fielding’s Department of Environmental Health Sciences, said in a statement.
“These findings raise concerns about indoor air quality post-wildfire, and the potential for prolonged exposure leading to significant health impacts.”
The research was published online Friday in the American Chemical Society journal Environmental Science & Technology Letters.
The January 2025 blazes in both the Pacific Palisades-Malibu area and the Altadena-Pasadena communities claimed at least 31 lives, burned more than 37,000 acres, and damaged or destroyed at least 18,000 structures, according to estimates by Los Angeles County.
Both fires erupted in open areas on Jan. 7, 2025, spread into neighborhoods, and were only fully contained by Jan. 31.
Researchers began field work on the second day of the wildfires, collecting air samples indoors and outdoors, and continuing through the post-fire period, ending Feb. 18, 2025. The team of UCLA faculty, staff and students were focused on the prevalence of fire-related pollutants, such as benzene, toluene, ethylbenzene, and xylenes.
“Some of these are carcinogenic and definitely harmful to human health,” said Dr. Yifang Zhu, professor in the Department of Environmental Health Sciences and a co-author of the study. “What made these urban wildfires particularly concerning was the potential toxicity of the resulting smoke, because (these compounds) are ubiquitous in residential areas, with common sources including building materials and household products, like cooking and cleaning supplies.”
The analysis found the presence of different volatile organic compounds peaked at different points during the fires and post-fire phases.
The presence of other compounds — toluene and n-hexane, for example — were found to have peaked in the later stage of active burning, both over the pre-fire background levels and, in some cases, the levels recorded during the blazes, which makes clear the risks of staying in a burn area, researchers said.
“In these cases, the indoor concentrations during active fires remained comparable to outdoor levels, indicating that staying indoors did not fully prevent exposure to fire-related pollutants,” said Dr. Yuan Yao, a UCLA Fielding researcher and first author of the study.
Mitigation measures can include regularly opening windows, operating central heating, ventilation and air conditioning systems, and using portable particulate air purifiers with charcoal filters when available, according to researchers.
The data make it clear that anyone living or working in burn areas — from firefighters to residents to construction workers — should understand the risks, the UCLA scientists said.