Residents returning to homes after wildfires may face prolonged exposure to cancer-causing chemicals that linger indoors even after outdoor air clears, according to a new UCLA study of the devastating January 2025 Los Angeles County blazes.
Researchers found that levels of some volatile organic compounds remained elevated inside smoke-damaged homes well into the post-fire recovery period, with certain toxic chemicals peaking weeks after flames were extinguished rather than during active burning.
The peer-reviewed study in Environmental Science & Technology raises concerns about indoor air quality in the aftermath of urban wildfires and suggests current safety guidance may be insufficient for protecting residents during recovery phases.
"Even after wildfires are extinguished, residents may remain at risk of exposure to indoor VOCs," said Dr. Yifang Zhu, a UCLA Fielding School of Public Health professor and study co-author. "This pattern suggests that smoke-impacted materials within the home could continue to release VOCs into the indoor environment, creating a prolonged source of exposure."
The research team, led by UCLA scientists, began collecting air samples Jan. 8, the second day of the fires that tore through Pacific Palisades-Malibu and Altadena-Pasadena communities. They continued monitoring through Feb. 18, tracking volatile organic compounds including benzene, toluene and other chemicals known to pose health risks.
The January blazes claimed at least 31 lives, burned more than 37,000 acres and damaged or destroyed more than 18,000 structures before being fully contained Jan. 31, according to county estimates.
Benzene, a known human carcinogen, showed particularly concerning patterns. Outdoor concentrations measured 0.38 parts per billion during active fires but remained elevated at 0.06 ppb weeks later — still significantly higher than pre-fire background levels that ranged from 0.00007 to 0.23 ppb.
More troubling, some compounds including toluene and n-hexane peaked during later stages of burning or the post-fire period, both indoors and outdoors, at levels exceeding those recorded during initial blazes.
"Our findings suggest that indoor air quality could remain affected even after outdoor smoke levels decline," said Dr. Yuan Yao, a UCLA researcher and the study's first author. "In other words, clearer skies outdoors may not immediately translate to 'clean air' indoors, especially in smoke-impacted homes."
The indoor persistence of these chemicals was particularly evident in uninhabited homes within burn zones, indicating ongoing emissions from smoke-damaged building materials and household items.
"Some of these are carcinogenic and definitely harmful to human health," Zhu said. "What made these urban wildfires particularly concerning was the potential toxicity of the resulting smoke, because VOCs are ubiquitous in residential areas, with common sources including building materials and household products."
Several compounds detected carry serious health risks. Besides benzene's cancer links, toluene and xylenes can irritate eyes and airways and affect the nervous system. N-hexane affects the nervous system, while carbon tetrachloride poses liver and kidney toxicity risks.
Children, older adults, pregnant women and those with chronic conditions like asthma or cardiovascular disease face heightened vulnerability, according to the research.
Dr. Michael Jerrett, UCLA professor and study co-author, said the findings "underscore the need for targeted interventions to minimize indoor exposures during the recovery phase."
Researchers recommend residents returning to affected homes improve ventilation by opening windows when outdoor air quality permits, use HVAC systems with MERV-13 or higher-rated filters, and run portable HEPA air cleaners with activated carbon filters. Ash and dust should be cleaned using wet methods or HEPA-filtered vacuums to prevent particles from becoming airborne.
The study is part of the ongoing Los Angeles Fire Human Exposure and Long-Term Health Study, supported by the Spiegel Family Fund, the Gordon and Betty Moore Foundation, the R&S Kayne Foundation and the California Air Resources Board.
The research involved UCLA faculty, staff and students working with the University of California, Davis, and the University of Southern California.