This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Caltech scientists have published the first detailed analysis of the atmospheric particles in the smoke plume from the Eaton Fire that devastated much of the town of Altadena in Southern California's San Gabriel Valley and the surrounding areas in January 2025. The results confirm the very different chemical nature of the smoke produced by an urban fire compared with the plume from a typical wildfire.
In the early stages of the fire, when many structures and vehicles were burning, the concentration of toxic metals in the atmosphere spiked dramatically. This chemical signature, which was measured by an advanced air-monitoring station located downwind of the fire, then evolved as the blaze moved from urban areas up into the mountains above Altadena, part of the Angeles National Forest. The scientists found that compared with wildfire smoke, the urban plume was enriched more than 50-fold in lead, sevenfold in chlorine and bromine, and threefold in arsenic.
In fact, based on the data, they estimate that at the peak of the Eaton Fire's structural burning, nearly 7 million people in the Los Angeles metropolitan area were exposed to 24-hour lead concentrations 80 times higher than typical values. These results are among the first to characterize the air that residents may be breathing during urban fires and could inform models used to determine emergency response to future urban fires. A paper describing the work was published on Oct. 28 in the journal Science Advances.
The lead author of the study is Haroula Baliaka (Ph.D. '27). "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 says.
"Smoke travels; it's not just about the burn area. What does a plume mean for people who are miles away from a fire but receive the smoke? We evacuate for fire, but we don't evacuate for smoke exposure." Baliaka worked with former Caltech graduate student and co-author Ryan Ward (Ph.D. '25) to set up an air-monitoring station in the city of Pico Rivera, located roughly 11 miles (18 kilometers) south of Altadena, that gathered data on the smoke plume throughout the Eaton Fire.
The station consists of a suite of four advanced analytical instruments that operate nearly 24 hours a day, 365 days a year, collecting data about the chemistry and physical properties of aerosols. Aerosols are the tiny particulate matter suspended in the air that we commonly hear about in air quality reports (though those typically only report on PM₂.₅—particles that are 2.5 microns or smaller in diameter). Colocated at an air quality monitoring station operated by the South Coast Air Quality Management District, the station is one of 12 around the country that make up the Atmospheric Science and Chemistry mEasurement NeTwork (ASCENT).
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