Caltech researcher warns Eaton Fire smoke left 7 million LA residents breathing 80x lead

California Institute of Technology researchers determined that nearly 7 million people in the Los Angeles metropolitan area breathed air with elevated lead concentrations during the peak of the Eaton Fire last year. The study, published in Science Advances, estimates that 24-hour lead levels reached 80 times typical values when the fire was at its height of structural burning. This finding marks the first detailed chemical characterization of air quality during an urban fire, a distinct phenomenon compared to standard wildfires.

The Eaton Fire killed 19 people, destroyed nearly 10,000 homes, and consumed more than 14,000 acres. Scientists noted that the smoke had a very different chemical nature than smoke from typical wildfires. During the early stages, when many structures and vehicles were burning, atmospheric concentrations of toxic metals spiked dramatically. An advanced air monitoring station located in Pico Rivera, approximately 11 miles south of Altadena, captured these changes as the blaze moved from urban areas into the Angeles National Forest.

Data from the Pico Rivera station showed the urban plume was enriched more than 50-fold in lead. Chlorine and bromine levels were seven times higher, while arsenic levels were three times higher than in a typical wildfire plume. Haroula Baliaka, a PhD student and lead author of the study, explained that these observational data allow scientists to adjust models to better estimate how smoke affects people miles away from a fire. She noted that while evacuations are standard for fire, exposure to smoke often goes unaddressed.

Baliaka co-authored the report with Caltech graduate student Ryan Ward. The two researchers used four instruments at the Pico Rivera station to continuously measure the chemistry and physical properties of airborne particles. This station is part of the National Science Foundation's Atmospheric Science and Chemistry mEasurement NeTwork, or ASCENT. Because ASCENT stations run continuously, the team could observe the plume's evolution as it moved from the city into the San Gabriel Mountains.

Sally Ng, a Caltech Professor of Environmental Science and Engineering and Chemical Engineering, emphasized that not all particles remained the same during the fire's progression. The type of particle in the air has different implications for toxicity. The study compared the Eaton Fire data with the Palisades Fire, which burned nearly 24,000 acres in Orange and Riverside counties. The comparison highlighted how chemically distinct an urban fire is, a fact Baliaka said might seem intuitive but lacks prior data support.

Lead levels spiked in eastern Los Angeles and the San Gabriel Valley on January 8 and 9. The highest readings were detected downwind of Altadena. As the fire moved into the mountains and approached typical wildfire levels by January 11, potassium ratios in the plume increased while lead levels decreased. Richard Flagan, a Caltech Professor of Chemical Engineering and Environmental Science and Engineering, argued that the response to the Eaton Fire was predicated on it being a wildfire event. He suggested it is time to reconsider fire codes to address urban firestorms in densely populated areas.

The report suggests that strong lead levels were unique to the Eaton Fire due to the amount of older homes and structures that burned in Altadena. In April 2025, Pasadena city leaders issued a public health advisory after finding lead levels exceeding federal screening guidelines. The Environmental Protection Agency also found lead in dozens of homes in early 2026. A University of California study released in June detailed that more than 3 million people downwind of the Palisades and Eaton fires were exposed to chromium-6, a metal associated with asthma, bronchitis, and lung cancer.

Dr. Howard Hu, a Keck Professor of Preventative Medicine at the University of Southern California, stated that health impacts depend on the dose of lead a person was exposed to. He said that for a single day of exposure, most people who did not become violently ill would likely be okay. His primary concern is chronic exposure spanning weeks, months, or years. Researchers aim to use the new data to update models that predict air quality and pollutant releases from urban fires.