Researchers from UCLA and UC Davis found airborne particles containing carcinogenic hexavalent chromium, also known as chromium-6, in neighborhoods near the burn scar areas.
The study, published in the journal Nature Communications Earth & Environment, found elevated concentrations of chromium-6 nanoparticles in the air around wildfire cleanup zones approximately two months after the fires were extinguished.
Researchers believe that the particles may have traveled up to nine miles downwind from the fire zones, reaching neighborhoods like Beverly Hills, West Hollywood and parts of the San Fernando Valley.
Chromium-6 is a toxic metal naturally found in soil, volcanic dust and gases.
The National Institute of Environmental Health Services found the compound is generally produced by industrial processes
Researchers say inhaling it can cause health problems.
"It can cause lung cancer, some throat and nasal cancers if the exposure is over a long period of time," said Dr. Michael Jerrett, a professor in UCLA's Department of Environmental Health Sciences and a co-author of the study. "On a more acute basis, you can see respiratory irritations, skin irritations, eye irritations."
Jerrett said the nanoparticles may have exposed as many as 3.3 million people to levels hundreds of times higher than those typically found in Los Angeles air.
Researchers reported average chromium-6 concentrations of 13.7 nanograms per cubic meter. While below federal workplace exposure limits, the levels exceeded U.S. Environmental Protection Agency screening thresholds for indoor air, according to the study.
Dr. Yifang Zhu, another UCLA co-author, said chromium-6 concentrations declined over time and returned to background levels roughly eight months after the fires as the substance converted into the less toxic chromium-3 form.
Researchers said the findings underscore growing concerns about health risks associated with wildland-urban interface fires as such events become more common.
"So if there is another fire that burns into an urban area, how should you treat that debris? And that's more, I think, the public health message from our study," said Michael Keeman, a professor in UC Davis' Civil and Environmental Engineering Department.
City News Service contributed to this report.