When fire tore through Los Angeles County in January 2025, westerly winds blew most of the smoke and ash over the Pacific, keeping the main measure of air quality, total mass of particles smaller than 2.5 microns, at or near normal levels.
But a study published in the Journal of Hazardous Materials found the particles floating while the fires burned differed sharply from ordinary urban pollution and from smoke generated mainly by burning vegetation. The samples contained enriched levels of chemicals in manufactured products: toxic metals, polycyclic aromatic compounds, volatile organic compounds and polyfluoroalkyl substances – also known as PFAS or "forever chemicals" – in the ash.
The particle mass of 2.5 microns "tells us how much particulate matter is in the air, but not what is in that particulate matter," said lead study author José Guillermo Cedeño Laurent , an assistant professor at Rutgers School of Public Health . "In a wildland-urban interface fire, you are not only burning trees. You are burning cars, batteries, wiring, metals, plastics and building materials."
Cedeño Laurent happened to be in Los Angeles for an environmental health and air-quality symposium when the fires began. He worked with colleagues in California to collect air and ash at a Pasadena residential site about 2.17 miles southwest of the Eaton Fire from Jan. 16 to Jan. 22.
The researchers collected size-segregated particulate matter, volatile organic compounds and ash. They then analyzed them for metals, polycyclic aromatic compounds, PFAS and other contaminants. The approach allowed them to examine ultrafine particles smaller than 100 nanometers, a fraction not systematically captured by standard air-quality monitoring and one that scientists suspect may be especially important to human health because such particles can penetrate deep into the lungs.
During the sampling period, the particle mass averaged 18.1 micrograms per cubic meter, which is below the national 24-hour standard of 35 micrograms per cubic meter for healthy air. Yet the chemistry told a different story. Ultrafine particles accounted for 40% of the particle mass. Noncrustal metals appeared mainly in the ultrafine fraction at about 30 times Los Angeles' normal levels and 10 to 1,000 times compared with biomass-only wildfire aerosols.
The team also found that the concentration of priority polycyclic aromatic hydrocarbons in particle mass exceeded normal levels by roughly tenfold. BTEX compounds – benzene, toluene, ethylbenzene and xylenes – were 4.8 to 13 times higher than urban background levels. Ash samples contained polycyclic aromatic hydrocarbons, metals and PFAS, raising concerns about contamination that could persist after flames are out.
Cedeño Laurent cautioned the study characterized pollutants rather than predicting how they would affect people. Also, the samples were limited to one site over two days. Still, the findings suggest that mass-based smoke readings can understate the potential hazard of fires that burn through densely built communities.
"I do not want the message to be simply scary," Cedeño Laurent said. "The point is that if we want to understand the risks, we need to know the composition of the particles, not just the amount."
That question is likely to become more pressing as more people live along the boundary between cities and wildlands. In the Los Angeles fires, the Palisades and Eaton fires together burned roughly 37,000 acres and destroyed more than 16,000 structures, turning everyday objects into a complex mixture of smoke and ash.
The next concern, Cedeño Laurent said, is what happens after the fire. Ash may look like debris to be removed but disturbing it during cleanup could send contaminated particles back into the air or carry them indoors, into soil or into water. Rutgers researchers are studying how much ash becomes airborne when it is disturbed and what particle sizes it produces.
"These fires leave a chemical legacy," Cedeño Laurent said. "To protect communities, we need monitoring and cleanup strategies that reflect what burned, not just how much smoke was measured."
