A recent study led by researchers at the University of California Los Angeles (UCLA) has found that smoke damage from wildfires can continue to pose health risks to residents even after the fires have been extinguished. The research, published in Environmental Science & Technology, focused on the 2025 wildfires in Los Angeles County and analyzed levels of volatile organic compounds (VOCs) both during and after the fires.
The team from UCLA Fielding School of Public Health discovered that VOC levels were significantly higher in homes during the post-fire period compared to when the fires were actively burning. These elevated concentrations were especially notable in uninhabited homes within burn zones, indicating ongoing indoor emissions from materials impacted by smoke.
“These findings underscore the need for targeted interventions to minimize indoor exposures during the recovery phase,” said Michael Jerrett, professor at UCLA Fielding’s Department of Environmental Health Sciences. “These findings raise concerns about indoor air quality post-wildfire, and the potential for prolonged exposure leading to significant health impacts.”
Researchers collected air samples indoors and outdoors beginning on January 8, 2025—the second day of two major blazes in Pacific Palisades-Malibu and Altadena-Pasadena—and continued through February 18, 2025. The study examined compounds such as benzene, toluene, ethylbenzene and xylenes (BTEX). According to their analysis, different VOCs peaked at various points during active burning and after fire containment. For example, median outdoor benzene concentrations were recorded at 0.38 parts per billion (ppb) during the fires but decreased to 0.06 ppb by the end of sampling.
“Some of these are carcinogenic and definitely harmful to human health,” said Yifang Zhu, a co-author and professor in environmental health sciences at UCLA. “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.”
The data also showed that indoor concentrations of certain pollutants remained similar to outdoor levels while fires burned actively—meaning staying inside did not fully protect against exposure. During recovery phases after fires ended, off-gassing from smoke-impacted materials appeared responsible for sustained high indoor VOC levels.
“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 Yuan Yao, first author of the study. “This indoor/outdoor pattern shifted during the post-fire recovery phase…which highlights the potential for prolonged indoor VOC exposure—even after outdoor fire activity subsides.”
To reduce risk following wildfire events, researchers recommend improving ventilation by opening windows regularly; using central heating or air conditioning systems equipped with filters rated MERV 13 or higher; and employing portable HEPA purifiers with charcoal filters if available.
“Even after wildfires are extinguished, residents may remain at risk of exposure to indoor VOCs,” Zhu said. “This pattern suggests that smoke-impacted materials within the home could continue to release VOCs into the indoor environment… To reduce these risks, residents returning to affected homes should adopt measures to improve ventilation and filtration.”
The work received support from faculty at UCLA as well as collaborators from UC Davis and USC. Funding came from several organizations including Spiegel Family Fund and California Air Resources Board as part of an ongoing effort called L.A. Fire HEALTH Study.
UCLA is recognized as a prominent public research institution offering diverse academic programs across a wide range of disciplines while supporting societal progress through education and research initiatives (official website). Its campus attracts students and scholars globally (official website)—including those who contribute expertise toward addressing issues like wildfire aftermath.
