When the Eaton Fire tore through Altadena in January 2025, the smoke looked like wildfire smoke. New research from Caltech shows it was chemically something else entirely — and that difference matters for the next urban fire.
Caltech scientists have published the first detailed analysis of the Eaton Fire’s smoke plume, in Science Advances on October 28, 2026. Compared with typical wildfire smoke, the urban plume was enriched more than 50-fold in lead, 7-fold in chlorine and bromine, and 3-fold in arsenic. At the peak of the fire’s structural burning, the researchers estimate, nearly 7 million people in the Los Angeles area were exposed to 24-hour lead concentrations 80 times higher than typical values.
Measured in real time, 11 miles downwind
The measurements came from an air-monitoring station in Pico Rivera, 11 miles south of Altadena. It is one of 12 sites in the Atmospheric Science and Chemistry mEasurement NeTwork (ASCENT), funded by the National Science Foundation, which measures airborne particle chemistry almost continuously.
That continuity made the study possible: the standard method collects particles on a filter over 24 hours, every few days, and analyzes them weeks later, so it misses spikes that come and go within hours. The Pico Rivera station watched the plume change hour by hour as the fire changed character. Lead author Haroula Baliaka set up the station with former graduate student Ryan Ward; the network is managed by Nga Lee (Sally) Ng, a corresponding author.
Two fires, two chemical fingerprints
To isolate what made the Eaton Fire’s smoke different, the team compared it with the Airport Fire, which burned about 24,000 acres of vegetation in Riverside County in September 2024. The contrast was starkest on January 8 and 9, 2025, when the Eaton Fire was burning structures and vehicles.
The researchers tracked the ratio of potassium to lead. Potassium comes from burning vegetation, so both fires showed elevated potassium. But in the Eaton Fire’s early, urban phase, the ratio sat around 5 to 1, climbing to about 50 to 1 by January 11 and into the hundreds by January 12 as the fire moved into the San Gabriel Mountains.
What this does — and does not — say about health
The study measured what was in the air, not lead in anyone’s blood, and the health effects of a short, intense burst of exposure like this one have not been established. “What do all of these short-term but elevated air toxins, not just lead, mean for our health?” Baliaka said. “We need to start figuring that out.”
The authors also caution that the Eaton Fire’s strong lead signature may be partly unique to this fire, because so many older homes burned in Altadena. The ratios should not simply be pasted onto the next urban fire.
Why it matters for Pasadena and Altadena
“The response to the Eaton Fire and to contamination of the debris left in its wake were predicated on the firestorm being a wildfire event,” said Richard Flagan, a Caltech professor of chemical engineering and environmental science and engineering. “But the Eaton Fire was an extreme urban fire event. Perhaps it is time to reconsider fire codes to address the possibility of urban firestorms in densely populated urban areas.”
Baliaka put it most directly: “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.”
Next, the researchers want the data in smoke models, so officials can judge what a downwind community is actually breathing in real time. Ng argues the country needs more stations like Pico Rivera’s: “With ASCENT, we are not only seeing how much particulate matter is in the air but what it is made of, and we can watch it as the event unfolds.”
Twenty-two months on, Altadena is still rebuilding and the smoke question is still open. This study does not close it — but it gives the region an hour-by-hour record of what was in the air as a neighborhood burned, and a reason to treat the next urban fire’s smoke as a different kind of emergency.
Sources: Caltech, “Chemical Data Collected from the Eaton Fire Plume Help Characterize Urban Fires,” October 28, 2026; Baliaka et al., “Fire at the boundary: Airborne toxic metals from the 2025 Los Angeles urban firestorm,” Science Advances, vol. 12, no. 44 (2026), DOI: 10.1126/sciadv.aei4455.
