When wildfires tore through Los Angeles in January 2025, thousands of burning homes, vehicles, and businesses released something much more hazardous than ordinary wildfire smoke.
As entire neighborhoods went up in flames, the fires sent enormous quantities of toxic metals and industrial chemicals into the atmosphere, creating a hazardous mixture that spread across the Los Angeles area.
Now, a new study reveals the extraordinary scale of that pollution. Researchers estimate that approximately 7 million people were exposed to airborne lead concentrations more than 80 times higher than typical background levels during the height of the disaster.
The findings, published in Science Advances, also reveal that smoke from burning neighborhoods contained more than 50 times the lead, seven times the chlorine and bromine, and three times the arsenic measured in a predominantly vegetation-fueled wildfire.
More concerning, the researchers discovered that conventional air-quality monitoring systems may be poorly equipped to identify these dangerous chemical exposures while fires are still burning.
“Smoke travels; it’s not just about the burn area,” lead author and Caltech PhD candidate Haroula Baliaka said in a press release. “We evacuate for fire, but we don’t evacuate for smoke exposure.”
When Homes Burn, Wildfire Smoke Becomes Something Different
On January 7, 2025, powerful Santa Ana winds drove the Eaton Fire through Altadena, destroying more than 9,000 homes and buildings.
Unlike conventional wildfires, where vegetation provides most of the fuel, the Eaton Fire consumed homes filled with synthetic furnishings, electrical equipment, plastics, and other manufactured materials. Moreover, many of Altadena’s older houses also date to periods when lead-based paint and brominated flame retardants were commonly used.
Researchers led by scientists at the California Institute of Technology analyzed continuous measurements collected by the Atmospheric Science and Chemistry Measurement Network (ASCENT), which tracks the chemical composition of fine airborne particles.
They compared pollution recorded during the Eaton Fire with measurements from a predominantly wildland fire near Riverside, California, in September 2024.
Surprisingly, both fires produced similar increases in the overall mass of fine particulate pollution, known as PM2.5. Their chemical makeups, however, differed dramatically.
The Los Angeles fire generated unusually high concentrations of lead, arsenic, copper, zinc, chlorine, and bromine, consistent with emissions from burning vehicles, electronics, building materials, and synthetic products.
In other words, two wildfires can produce nearly identical levels of airborne particulate pollution while exposing nearby populations to vastly different concentrations of toxic chemicals.
Scientists Tracked the Fire’s Changing Chemistry Hour by Hour
One particularly interesting discovery involved the relationship between two elements: potassium and lead.
Potassium is commonly associated with burning vegetation, while elevated lead concentrations can indicate combustion of manufactured materials.
During the Eaton Fire’s initial assault on residential neighborhoods, researchers recorded potassium-to-lead ratios as low as 5.
In the predominantly wildland fire used for comparison, those ratios typically ranged from approximately 50 to 500.
As the Eaton Fire moved away from developed neighborhoods and into the San Gabriel Mountains, the ratio steadily increased toward values associated with vegetation fires.
Essentially, scientists could track the transition from burning homes to burning vegetation through changes in the smoke’s chemical fingerprint.
The finding shows that watching these elements in real time could help emergency officials recognize when a wildfire begins producing particularly hazardous urban pollution.
Millions Exposed to a Toxic Cloud
The researchers’ most consequential finding involved the enormous population potentially exposed to airborne lead.
On January 9, approximately 7 million Los Angeles-area residents experienced estimated 24-hour average lead concentrations exceeding 50 nanograms per cubic meter, roughly 80 times the typical level measured at the Pico Rivera monitoring site.
The study’s uncertainty range placed the affected population between 4.3 million and 8 million people.
At one monitoring location, hourly lead concentrations peaked at 470 nanograms per cubic meter.
That was more than three times the Environmental Protection Agency’s ambient lead standard of 150 nanograms per cubic meter. However, the EPA standard is based on a three-month rolling average rather than hourly measurements.
However, the brief spike cannot be considered a violation of EPA standards, which are based on a three-month rolling average. Nor does it establish what health consequences, if any, resulted from the exposure.
Nevertheless, lead is a well-established neurotoxin, markedly concerning for children, while arsenic is associated with increased cancer risks.
The study did not measure individual lead absorption or establish whether the exposures caused specific illnesses.
Researchers also emphasized that the health consequences of short-duration exposure to such concentrated mixtures remain poorly understood.
A Hidden Danger That Can Outlast the Flames
The findings expose a potential weakness in existing wildfire emergency responses.
Conventional air-quality networks can rapidly measure particulate pollution, but often cannot identify its chemical components until laboratory analysis is completed weeks or months later. As a result, authorities may know how much smoke is present without knowing whether it contains unusually high concentrations of toxic metals.
The danger also goes beyond the immediate fire. Airborne contaminants can travel considerable distances before settling onto playgrounds, agricultural land, reservoirs, and surrounding communities. Wind and rain can subsequently redistribute contaminated ash.
The researchers argue that emergency monitoring systems must begin distinguishing between vegetation smoke and pollution generated by burning urban infrastructure.
Ultimately, as wildfires increasingly threaten populated areas, the findings show that measuring atmospheric smoke may no longer be enough—knowing exactly what is burning could be just as important as knowing how much smoke it produces.
The study, “Fire at the boundary: Airborne toxic metals from the 2025 Los Angeles urban firestorm,” appeared in Science Advances.
Tim McMillan is a retired law enforcement executive, investigative reporter and co-founder of The Debrief. His writing typically focuses on defense, national security, the Intelligence Community and topics related to psychology. You can follow Tim on Twitter: @LtTimMcMillan. Tim can be reached by email: tim@thedebrief.org or through encrypted email: LtTimMcMillan@protonmail.com