A new California Institute of Technology study reveals that smoke from the devastating 2025 Eaton Fire contained sharply elevated concentrations of lead, arsenic and other toxic substances, exposing millions of Los Angeles residents to pollution levels far above normal.
By yourNEWS Media Newsroom
LOS ANGELES — Nearly 7 million people across the Los Angeles metropolitan area were exposed to airborne lead concentrations approximately 80 times higher than normal during a 24-hour period at the height of the January 2025 Eaton Fire, according to a new study examining the toxic chemicals released when wildfires destroy homes and other structures.
The research, published Wednesday, Oct. 28, in the journal Science Advances, found that smoke from the Eaton Fire contained substantially higher concentrations of hazardous metals and chemical compounds than smoke produced by fires burning primarily through vegetation.
Scientists at the California Institute of Technology determined that the urban fire plume contained more than 50 times as much lead, seven times as much chlorine and bromine, and three times as much arsenic as smoke from a predominantly wildland fire.
The findings, detailed in a Caltech announcement, highlight the additional health hazards created when wildfires spread into residential communities, burning vehicles, buildings, household chemicals and electronic equipment.
“The main, key finding is really that wildfire smoke is very different fundamentally from the urban smoke,” said Nga Lee “Sally” Ng, a study co-author and Caltech environmental science and engineering professor.
Ng said advanced monitoring technology allowed scientists to “actually see the very detailed component of these particles that we are breathing.”
The Eaton Fire began Jan. 7, 2025, in the foothills above Altadena, northeast of Los Angeles. Strong winds rapidly pushed the flames into residential neighborhoods, destroying thousands of homes and businesses.
The Eaton and Palisades fires together killed at least 31 people and destroyed more than 16,000 structures, making them among the most destructive disasters in California history.
Researchers said the destruction of manufactured materials created a chemically different type of smoke than that produced when forests, grasslands and other vegetation burn.
To investigate those differences, scientists analyzed measurements collected at an air-monitoring station in Pico Rivera, approximately 11 miles south of Altadena.
The station belongs to the Atmospheric Science and Chemistry mEasurement NeTwork, known as ASCENT, a National Science Foundation-funded network of 12 monitoring locations.
Unlike conventional air-quality monitoring systems that may collect samples only periodically, ASCENT equipment can measure the chemical composition of fine airborne particles, known as PM2.5, at hourly intervals.
That capability allowed researchers to track changes in smoke composition as the Eaton Fire moved through developed neighborhoods and into surrounding mountain areas.
The highest concentrations of lead occurred Jan. 8 and 9, when large numbers of homes and vehicles were burning.
Using those measurements and population data, researchers estimated that approximately 7 million residents experienced 24-hour average lead concentrations more than 80 times typical background levels.
The estimate describes elevated airborne lead exposure, not confirmed lead poisoning among those residents.
Scientists compared the Eaton Fire measurements with data from the 2024 Airport Fire in Riverside County, which burned predominantly through undeveloped vegetation.
The comparison revealed that urban smoke contained dramatically higher concentrations of several potentially harmful substances.
Lead can originate from older building materials, paint and other manufactured products. Arsenic and other metals may be released when treated materials, vehicles and household contents burn.
The researchers also identified substantial increases in chlorine and bromine, chemical elements associated with certain plastics, flame retardants and other manufactured materials.
Study lead author Haroula Baliaka said the findings demonstrate why urban fires require different approaches to understanding smoke pollution.
“Urban fire plumes are chemically distinct from those of wildfires. Now that we have these observational data about how they differ, we can go back and adjust our models to more accurately estimate how smoke might affect people,” Baliaka said.
Researchers also observed that the chemical composition of the smoke changed as the fire progressed.
During the initial destruction of Altadena neighborhoods, lead concentrations increased sharply relative to potassium, an element commonly released when vegetation burns.
As the flames moved toward forested mountain areas, lead concentrations declined relative to potassium, indicating a transition toward a more typical wildfire smoke signature.
The changing chemical measurements helped researchers distinguish pollution associated with burning structures from emissions produced by burning vegetation.
The health implications remain an important unanswered question.
Lead is a neurotoxin associated with impaired neurological development in children, reduced cognitive performance and increased blood pressure in adults. Arsenic exposure has been linked to several cancers.
However, much of the existing research examines prolonged or repeated exposure rather than brief periods involving unusually high concentrations of multiple toxic substances.
Scientists have not established the precise long-term health consequences of the 24-hour exposure levels documented during the Eaton Fire.
Ed Avol, professor emeritus at the University of Southern California’s Keck School of Medicine, said the findings were consistent with concerns about heavy metals released during urban fires.
The results were “certainly surprising because we haven’t been able to document this sort of thing,” said Avol, who was not involved in the study.
He emphasized the value of having detailed measurements available throughout the fire.
“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,” Avol said.
The research also reinforces concerns raised by previous studies examining the health effects of smoke, ash and debris after destructive urban fires, including the 2023 Maui wildfires.
Yaguang Wei, an environmental medicine researcher at the Icahn School of Medicine at Mount Sinai, said the findings were consistent with evidence suggesting that smoke from fires involving manufactured materials can contain particularly harmful pollutants.
“I think their findings are not surprising,” Wei said.
He argued that governments should invest in improved air monitoring, public warning systems and strategies to reduce wildfire risks.
Researchers noted that expanding development near fire-prone vegetation, combined with hotter and drier conditions associated with climate change, increases the potential for destructive fires where urban and natural environments meet.
The findings suggest that emergency response systems may need to account for the chemical composition of smoke, rather than relying exclusively on measurements of overall particulate pollution.
Baliaka said the research could help improve models used to estimate hazardous exposure and guide public health decisions during future fires.
The study provides evidence that people living far beyond the immediate fire perimeter may encounter hazardous airborne substances when residential neighborhoods burn.
Although the long-term consequences of the Eaton Fire exposures remain uncertain, the researchers said the findings demonstrate the importance of expanding real-time chemical monitoring and adapting emergency planning to the distinctive dangers of urban wildfire smoke.
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