What Wildfires Leave Behind

An analysis of soils and ash in residential Los Angeles areas showed varying levels of contamination by toxic elements

an aerial view of the wildfires
A drone captured structures damaged by the Eaton Fire in Altadena, Calif., on Jan. 29, 2025. Photo by Ringo Chiu/Shutterstock

This is a summary of a story on Nicholas School of the Environment. 

Samples of soil and ash collected from residential areas in Los Angeles that were burned by wildfire revealed wide variation in contamination, according to a new study. The study offers insight into Southern California’s urban history and the effectiveness of post-fire cleanup efforts. 

In January 2025, wildfires blazed across the L.A. communities of Altadena and Pacific Palisades, killing 31 people and destroying more than 16,000 structures, most of which were homes.

“Homes often contain metals in structural materials and household products, and fires release metals in smoke but also concentrate metals in structural ash that can potentially contaminate residential soils,” says Daniel Richter, a soil scientist at the Nicholas School of the Environment, who oversaw the study.

Richter’s team analyzed samples of soil and ash collected by volunteers in Los Angeles shortly after the fires. The researchers found highly variable levels of lead and arsenic across both sample types. 

Lead levels were highest in the structural ash from a fraction of homes built before the 1970s, likely from lead-based paints used at the time, according to the team.

That summer, after a federal cleanup effort, the L.A.-based team gathered additional samples in the vicinity of the original sites. 

Analysis by Richter’s lab showed greatly reduced concentrations of lead and arsenic in samples from burned areas that had been scraped by bulldozers during the cleanup effort, especially at sites that had shown the highest concentrations of those contaminants.

“These data are especially important because they are some of the only comparisons of pre- and post-cleanup,” Richter says.

The study highlights an area of disagreement regarding cleanup effectiveness, according to Richter. California is currently the only state with a state-set level for lead in residential soil. That level is less than half the federal standard set by the U.S. Environmental Protection Agency.

“The two lead concentrations needlessly confuse the public, and they critically need a technical review,” Richter says. 

For more information on the wildfire study, go to Nicholas School of the Environment.