Study finds 7 million exposed to high lead levels in Los Angeles fire smoke

Hourly monitors tracked chemical changes during the Eaton Fire

A California Institute of Technology study found that at the height of the 2025 Los Angeles fires, about 7 million people were exposed for 24 hours to lead levels more than 80 times the typical background level. Published Wednesday in the journal Science Advances, the study examined the chemical composition of smoke from the Eaton Fire and estimated how many people across Los Angeles were exposed to heavy metals during the fire’s first days.

The researchers compared the Eaton Fire plume with smoke from a 2024 Riverside County fire that burned mostly vegetation and with the typical level of lead before the fires. They found that the plume from the Los Angeles fire contained 50 times more lead, three times more arsenic and seven times more halogens. The researchers linked those components to burning vehicles, building materials and electronics. Halogens include chlorine and bromine, according to the report.

Nga Lee “Sally” Ng, an author of the study, said, “The main, key finding is really that wildfire smoke is very different fundamentally from the urban smoke.” Ng said the technology used in the study allowed researchers to “actually see the very detailed component of these particles that we are breathing.”

Researchers measured trace elements in fine particulate matter, or PM2.5, every hour through instruments in the Atmospheric Science and Chemistry mEasurement NeTwork, a relatively new network of air-quality monitors across the U.S. The network measures the size and chemical composition of the particles in real time. Ng said ASCENT is the U.S.’s first measurement network able to measure the chemical composition of airborne particles in real time.

The measurements allowed researchers to track how the smoke’s composition changed over the fire’s course, distinguish emissions coming from burning structures and estimate how many people across Los Angeles were exposed to heavy metals, particularly lead, during the first days.

The two wind-whipped infernos in Altadena and the Pacific Palisades ignited on Jan. 7, 2025. They killed at least 31 people and destroyed more than 16,000 structures, mostly homes, along with the cars, paint, furniture, chemicals under sinks and other contents inside them. Millions of people were exposed to the smoke.

Wildfire smoke across the United States has been linked to tens of thousands of deaths annually, the report said. A recent study also found that metal mixtures in leftover smoke, ash and debris from the deadly 2023 Maui wildfires were harmful to the lung health of people living nearby long after the burning ended.

The report described lead as a neurotoxin that can stunt children’s development, lower IQ scores and increase blood pressure in adults. It said scientists still do not know how short-term, high exposure to lead affects human health. The report also said arsenic exposure is associated with various cancers.

Ed Avol, a professor emeritus in the Keck School of Medicine at the University of Southern California who has researched the health effects of air pollution for decades, said significant exposure to heavy metals was consistent with what he expected, “but certainly surprising because we haven’t been able to document this sort of thing.” He also called the study’s sampling an advantage: “One of the real advantages of this study is that it had the opportunity to make measurements sort of hour-by-hour across the sampling network. And typically we make measurements of airborne particulates and so forth a couple of times a week, at best.”

Yaguang Wei, an assistant professor in the department of environmental medicine at Icahn School of Medicine at Mount Sinai who researches the health impacts of air pollution, wildfire smoke and severe weather, said the findings reinforced a recent study he co-authored. That study found, in part, that PM2.5 from wildfire smoke was more toxic than particulates from other sources.

“I think their findings are not surprising,” Wei said. When fires burn properties, they also burn “a lot of lead on your wall paint and the plastics and electronics and those are very toxic,” he said.

Wei said the study has “important policy implications.” He thinks governments should take stronger actions to protect public health during fires by investing in more wildfire monitoring and public alerts, and by having more prescribed fires. Avol, separately, described a “strange and increasingly dangerous new world” in which urban and wildland areas intersect and increasingly catch fire.

“It reminds us to be cautious and be concerned and, you know, think about what we can do to minimize this. We’re not going to stop using all these materials containing metals and halogens and so forth in our homes,” Avol said. “How do we protect people from being exposed to that?”

Co-author Haroula Baliaka said understanding the different chemical compositions of smoke from an urban versus a wildland fire helps bridge a missing gap in research. The report said wildfires are growing larger, more intense, faster-moving and more frequent largely because of drier and warmer weather associated with human-caused climate change. As urban growth and climate change push more fires into cities, researchers said monitoring and emergency response will have to adapt.