• Posted May 27, 2026

Ultrafine Wildfire Smoke Particles May Pose Serious Health Risks

Wildfire smoke carries a wide array of potentially toxic particles, in sizes so small they could prove a threat to human health, a new study says.

Smoke samples taken during the Los Angeles wildfires of 2025 contained high levels of toxic metals, polycyclic aromatic compounds, volatile organic compounds and PFAS “forever chemicals,” researchers are reporting in the July issue of the Journal of Hazardous Materials.

Many of these compounds are known to be toxic, causing cancer and contributing to other health problems.

“In a wildland-urban interface fire, you are not only burning trees. You are burning cars, batteries, wiring, metals, plastics and building materials,” said lead researcher José Guillermo Cedeño Laurent, an assistant professor at Rutgers School of Public Health in New Brunswick, New Jersey.

Cedeño Laurent was in Los Angeles in January 2025 for an environmental health and air quality symposium when wildfires tore through that region.

He worked with colleagues to collect air and ash samples at a Pasadena home a little more than two miles southwest of one of the wildfires, between Jan. 16 and 22.

The team then analyzed the samples, trying to break down the chemicals contained in the soot and ash.

Typically, environmental health experts and regulators look at air pollution particles that are less than 2.5 microns in diameter. By comparison, a human hair is 50 to 70 microns wide, according to the Arctic Monitoring and Assessment Program.

But these researchers examined ultrafine particles that were smaller than 100 nanometers – and one micron is equal to 1,000 nanometers.

These ultrafine particles accounted for 40% of the total mass of the samples, researchers found. 

Airborne toxic metals were 30 times Los Angeles’ normal levels and up to 1,000 times greater than smoke from wildfires that burn solely trees and vegetation.

Polycyclic aromatic hydrocarbons (PAH) were roughly 10 times higher than normal levels. These included benzene, toluene, ethylbenzene and xylenes that were 4.8 to 13 times higher than urban background levels.

PAH chemicals have been linked to cancer, asthma and damage to the immune and reproductive systems, according to Oregon State University.

The samples also showed PFAS and other chemicals blended into particles from burned vegetation, researchers said.

“I do not want the message to be simply scary,” Cedeño Laurent said in a news release. “The point is that if we want to understand the risks, we need to know the composition of the particles, not just the amount.”

Worse, these contaminated particles pose a double risk, researchers said – first when they are airborne, and again when they are disturbed during cleanup. Efforts to pick up the pieces following a devastating wildfire might wind up spreading toxins into the air, soil, water or indoors.

“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.”

More information

The U.S. Environmental Protection Agency has more on particle air pollution.

SOURCE: Rutgers University-New Brunswick, news release, May 21, 2026