Smoke from the Eaton Fire had a sharply different chemical makeup from typical wildfire smoke, with toxic-metal levels spiking while homes and vehicles burned, according to a Caltech study published Wednesday.
Researchers estimated that nearly 7 million people in the Los Angeles metropolitan area were exposed to 24-hour lead concentrations 80 times higher than typical values at the peak of the fire’s structural burning. The estimate was based on data from an air-monitoring station in Pico Rivera, about 11 miles south of Altadena, that tracked the fire plume.
The study, published in the journal Science Advances, found that the urban plume contained more than 50 times as much lead, seven times as much chlorine and bromine, and three times as much arsenic as typical wildfire smoke.
“Smoke travels; it’s not just about the burn area,” lead author Haroula Baliaka said. “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 and co-author Ryan Ward monitored the plume at the Pico Rivera station. Four instruments continuously measured the chemistry and physical properties of airborne particles, allowing researchers to track rapid changes that can be missed by standard sampling methods, which collect particles over 24-hour periods every few days and analyze them weeks or months later.
The researchers also compared the Eaton Fire data with measurements from the Airport Fire, which burned 24,000 acres of vegetation in Riverside County in September 2024.
As the Eaton Fire moved from developed areas into the San Gabriel Mountains, its chemical signature shifted toward that of a more typical wildfire, the study found.
“Essentially, this comparison between the two fires indicates how different and chemically distinct an urban fire is,” Baliaka said.
Lead levels spiked across eastern Los Angeles and the San Gabriel Valley on Jan. 8 and 9, with the highest readings downwind of Altadena. The ratio of potassium to lead in the plume increased as the fire moved into the mountains, approaching wildfire-like levels by Jan. 11 and rising further the next day, according to the study.
Researchers said the health effects of short-term exposure to high concentrations of lead and other toxic chemicals remain unknown. The findings could help improve models used to predict where smoke and pollutants will travel during fires and guide emergency responses, according to Caltech.