Nearly 7 million people in greater Los Angeles may have encountered airborne lead concentrations about 80 times typical levels over 24 hours during peak structural burning in the January 2025 Eaton Fire, Caltech researchers estimate. Their study, published in Science Advances on October 28, 2026, examines pollution exposure rather than establishing health outcomes for those residents.
The team compared the Eaton plume with smoke from the September 2024 Airport Fire in Riverside County, which largely burned vegetation. Caltech reports that the urban smoke contained more than 50 times as much lead, seven times as much chlorine and bromine, and three times as much arsenic. These comparisons concern wildfire smoke, while the 80-fold figure uses normal background lead levels.
Four instruments at an ASCENT monitoring station in Pico Rivera, roughly 18 kilometres south of Altadena, tracked particles continuously. According to Caltech, this approach captured rapidly changing pollution that conventional filters, collected periodically and analysed later, can miss. The measurements followed the plume as the fire moved from developed areas into the mountains.
Lead author Haroula Baliaka said the findings could improve models of smoke exposure beyond the burned area. The researchers observed that the plume became more like vegetation-fire smoke as the blaze moved away from buildings. They caution that Altadena's older structures may help explain the strong lead signal, limiting how broadly it can be applied to other urban fires.
Caltech says the health effects of brief exposure to such high concentrations of multiple toxic substances remain uncertain. The research documents an environmental exposure estimate, not a count of people diagnosed with lead poisoning. The Associated Press independently reported the findings on October 28, highlighting the different chemical composition of smoke from burning urban materials.
The researchers want to incorporate these observations into pollution forecasts used during fire emergencies. Caltech's Nga Lee Ng also called for more continuous monitoring to show both the amount and composition of airborne particles. Better measurements could help emergency managers assess what communities downwind are breathing as a fire develops.
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