About 7 million people across Los Angeles breathed smoke carrying lead at more than 80 times normal levels for a full day during the January 2025 Eaton Fire, according to a new study from the California Institute of Technology. The research, published Wednesday, Oct. 28, in Science Advances, found that smoke from burning homes, cars, and electronics carried far more toxic metal than smoke from fires that burn mostly brush and trees.

The finding matters well beyond Altadena. Millions of Americans live where neighborhoods meet wildland, and the study suggests that when a fire moves into a city, the smoke is no longer just a vegetation problem. It becomes a chemical exposure — one that existing air-quality alerts were not designed to describe.

The researchers stress what they do not yet know. The health effects of a short, intense burst of lead exposure like this one have not been established, and the study measured air concentrations, not levels in people's blood.

Smoke Unlike a Typical Wildfire

The Caltech team, led by atmospheric chemists Haroula Baliaka and Nga Lee "Sally" Ng, used a monitoring station in Pico Rivera, south of the fire, that reports the chemical makeup of fine particles every hour. Under normal conditions, daily lead there averages about 0.6 nanograms per cubic meter. On Jan. 9, 2025, about 7 million residents experienced lead concentrations above 50 nanograms per cubic meter, ScienceAlert reported.

To test whether the difference came from fuel, the researchers compared the Eaton plume with smoke from a 2024 Riverside County wildfire that burned almost entirely through vegetation. The Eaton plume carried about 50 times more lead, seven times more chlorine and bromine, four times more zinc, and three times more copper and arsenic.

The team traced the gap to what burned. More than 9,000 structures were destroyed in the Eaton Fire, along with vehicles and electronics. Many Altadena homes date from the early to mid-20th century, when lead paint and brominated flame retardants were common.

"The main, key finding is really that wildfire smoke is very different fundamentally from the urban smoke," Ng said, according to the Associated Press.

Neighborhoods That Breathed the Most Lead

The researchers mapped daily lead levels at roughly 1-kilometer resolution from Jan. 7 to Jan. 11, 2025, then overlaid those maps with census data. Air quality was worst during the first two days. Eastern Los Angeles and the San Gabriel Valley recorded the highest 24-hour average lead concentrations, and the total number of exposed people kept rising as the plume spread across the basin.

That pattern means the exposure was not limited to people who lived inside the burn zone. Residents miles downwind, including many who never evacuated, may have breathed metal-laden smoke without any warning that its chemistry differed from ordinary wildfire smoke.

The measurements came from the Atmospheric Science and Chemistry Measurement Network, known as ASCENT, a relatively new national network that Ng described as the first in the United States able to measure the chemical composition of airborne particles in real time. A separate UCLA study using stations in Westwood and Huntington Park reported similar results.

Independent Scientists Weigh the Findings

Ed Avol, a professor emeritus at the University of Southern California's Keck School of Medicine who was not involved in the study, said the hour-by-hour data was a major advance, since particle samples are typically collected a few times a week at best. He said significant metal exposure was consistent with what he would expect, "but certainly surprising because we haven't been able to document this sort of thing."

Yaguang Wei, an assistant professor of environmental medicine at the Icahn School of Medicine at Mount Sinai, also not involved, said the results were not surprising given what burns inside homes. He said the study carries policy implications, including more investment in wildfire monitoring, public alerts, and prescribed fire.

On the evidence itself, this is an observational air-monitoring study of one urban fire, compared with one vegetation fire. It establishes what was in the air and roughly how many people were exposed to it. It does not show how much lead people absorbed or whether anyone was harmed, and the population estimate depends on mapping from a limited number of monitors. Lead is a neurotoxin with no known safe level for children, and arsenic is linked to several cancers, but the study authors called for epidemiological research to measure actual health outcomes.

Protecting Households During the Next Urban Fire

For families, the practical lesson is that urban fire smoke deserves more caution than a hazy day from a distant forest fire. When fires burn homes nearby, health officials generally advise staying indoors with windows closed, running a portable HEPA air cleaner or a central system with a high-efficiency filter, and wearing a well-fitted N95 respirator outdoors. Cloth and surgical masks do little against fine particles.

Children, pregnant people, older adults and people with heart or lung disease face the greatest risk. Parents in Los Angeles County who are worried about past exposure can ask a pediatrician about blood lead testing, which the county has offered for fire-affected residents.

The contamination does not end when the smoke clears — metals can linger. Researchers noted that metals may persist in soil, playgrounds and waterways. Nature World News previously reported on lingering questions about toxic soil in rebuilt Los Angeles neighborhoods, and this study adds an airborne piece to that picture.

The bottom line: the Eaton Fire's smoke carried far more lead and other metals than typical wildfire smoke, reaching about 7 million people. The health effects remain unknown, and the researchers say the next step is studying those exposed. Until then, residents near any fire that is burning homes should treat the smoke as a chemical hazard and limit exposure.

What Readers Want to Know

What did the study find? Smoke from the Eaton Fire exposed about 7 million people to lead levels more than 80 times normal for 24 hours, with far more metals than smoke from a vegetation-only wildfire.

Which areas had the highest lead levels? Eastern Los Angeles and the San Gabriel Valley recorded the highest 24-hour average concentrations, mainly during the fire's first two days.

Does this mean people were poisoned? The study measured air, not blood. Scientists do not yet know the health effects of short, intense lead exposure, and the authors called for follow-up health studies.

Why was the smoke so different? The fire burned more than 9,000 structures, plus cars and electronics. Older homes contained lead paint and flame retardants that released metals and halogens when burned.

How can households protect themselves during an urban fire? Stay indoors with windows closed, use HEPA filtration, wear a fitted N95 outdoors, and follow local health officials' smoke guidance.

Should my child get a lead test? Parents worried about exposure should ask their pediatrician, who can order a simple blood lead test.