A cancer-causing metal detected in the air around last year's Los Angeles wildfire cleanup zones may have exposed more than 3 million residents to dangerous levels of a toxic carcinogen, according to a new study by University of California researchers.
The peer-reviewed study, published in the journal Nature Communications Earth & Environment, found hexavalent chromium — commonly known as chromium-6 — in neighborhoods surrounding the Palisades and Altadena fire zones two months after the January 2025 blazes. Calculations using models for wind-carried contaminants suggest the microscopic particles traveled as far as six to nine miles from the burn areas, reaching communities including Beverly Hills, West Hollywood and parts of the southern San Fernando Valley.
Researchers estimate approximately 3.3 million people received doses of the carcinogen at levels hundreds of times higher than those normally found in Los Angeles air.
"It shows that well after the wildfires were extinguished, nanoparticles, which are so small they can enter the circulatory system very quickly, were in the air around the burn zones," said Dr. Michael Jerrett, professor in the Department of Environmental Health Sciences at the UCLA Fielding School of Public Health and a co-author of the study. "These probably traveled far enough to give 3.3 million people doses that were hundreds of times the levels that are normally seen in the air in Los Angeles."
Chromium-6 is a toxic metal linked to asthma, bronchitis and lung cancer. The nanoparticles identified in the study measure less than one-thousandth the width of a human hair — small enough to cross cell membranes and move through the bloodstream.
Researchers measured average concentrations of 13.7 nanograms per cubic meter in the cleanup zones. While those levels fall below the National Institute for Occupational Safety and Health's workplace exposure limit of 200 nanograms per cubic meter, they significantly exceed the U.S. Environmental Protection Agency's screening levels for indoor air — 0.1 nanograms per cubic meter for cancer risk and 3 nanograms per cubic meter for non-cancer health effects.
Lead author Dr. Michael Kleeman, a professor at UC Davis, said the findings were released early to alert residents and other researchers, even as the study undergoes final revisions.
"Caution and health surveillance is warranted for nearby residents given that nanoparticles can easily cross cell membranes and circulate throughout the body," Kleeman said. "Discovering these airborne, chromium-bearing nanoparticles in the wildfire debris cleanup zones is a unique finding that implicates the fires as a source of toxic nanoparticle metals."
The source of the elevated chromium-6 remains unknown. Researchers point to a range of possible culprits in the burned structures, including computer screens, solar panels, electric vehicles and flame retardants — all materials common in the urban neighborhoods consumed by the fires.
The study also raises a concerning timeline question: monitoring did not begin until two months after the fires were extinguished. Jerrett said the delay was due to a lack of funding, with researchers needing time to secure internal support before fieldwork could begin.
Children, the elderly and people with pre-existing health conditions are generally more vulnerable to the effects of air pollution, Jerrett noted, though he cautioned that the novelty of this type of nanoparticle exposure means researchers do not yet know whether those groups faced disproportionately greater harm.
There is some encouraging news. Co-author Dr. Yifang Zhu, a UCLA professor of environmental health sciences, said chromium-6 concentrations declined over time and returned to background levels approximately eight months after the fire, as the carcinogenic form converted to the less toxic chromium-3.
"This highlights the importance of continued monitoring to track how environmental risks change during the recovery process," Zhu said.
Researchers are urging residents living near wildfire cleanup zones to use indoor air filters and limit outdoor exercise in affected areas until conditions return to safe levels.
The January 2025 fires killed at least 31 people and damaged or destroyed more than 18,000 structures across Los Angeles County, according to county figures. The blazes swept through the Pacific Palisades-Malibu area in western Los Angeles and the Altadena-Pasadena foothills of the San Gabriel Mountains.
Scientists warn the findings carry implications far beyond Los Angeles, given the increasing number of fires that begin in open space and cross into urban areas.
"Unfortunately, given continued expansion of wildland-urban interface areas worldwide alongside increasing wildfire risk, we are likely to see more and more of these sorts of fires — and to deal with their impacts — in the future," said Dr. Christopher Cappa, a UC Davis environmental engineering professor and study co-author.
The study received partial funding from the Speigel Foundation, with additional support from UC Davis and UCLA.