The January wildfires left many scars on the city of Los Angeles, from rubble-reduced homes to torched abandoned vehicles. Though cleanup crews quickly cleared much of the debris, one alarming invisible impact lingered over the city for months, a new study suggests.
In late March—more than two months after the flames died out—researchers detected levels of carcinogenic hexavalent chromium (a.k.a. chromium-6) 200 times greater than baseline levels for LA air. If this pollutant sounds familiar, you’re probably thinking of the 2000 film Erin Brockovich, a dramatization of a true story about hexavalent chromium water contamination. Though the levels the researchers detected fell below certain safety thresholds, the particles’ unusually small size immediately raised concerns.
The study is currently available on the preprint server Research Square, but it has been reviewed by the LA Health Consortium, lead author Michael Kleeman, an environmental engineer at the University of California Davis, told Gizmodo in an email. Though it has yet to go through formal peer review, he and his colleagues chose to release the findings to alert policymakers and the public to this potentially hazardous pollutant as soon as possible.
In a statement to Science Magazine, the South Coast Air Quality Management District emphasized that the study’s sampling was limited and that its own data do not suggest there is an immediate health risk from hexavalent chromium.
Fire activates chromium’s toxicity
Chromium is a heavy metal that naturally occurs in soil, plants, and rocks, but it’s also present in some building materials, including stainless steel, chrome plating, pigments, and cement. In its common form, chromium III is an essential nutrient that helps the body break down fats and carbohydrates.
When oxidized, chromium III becomes toxic hexavalent chromium. Certain levels of exposure to this pollutant may increase the risk of lung, nasal, and sinus cancer, according to the Occupational Safety and Health Administration. Research has shown that fire can drive the oxidation of chromium III, and a 2023 study found that hexavalent chromium can be present in wildfire smoke and ash.
Thus, Kleeman and his colleagues expected to find hexavalent chromium when they sampled air from debris cleanup zones around the Eaton and Palisades fires. They detected concentrations ranging from 8.1 nanograms to 21.6 nanograms per cubic meter in the neighborhoods most affected by the fires: Altadena and the Pacific Palisades. This is well below the National Institute for Occupational Safety and Health’s workplace exposure limit of 200 nanograms per cubic meter of air but above the EPA’s indoor limit of 0.1 nanogram per cubic meter.
What they didn’t expect was the puny size of the particles. “It is really surprising to find all of the hexavalent chromium in the LA fire debris cleanup zones concentrated in particles smaller than 56 nanometers,” Kleeman said.
Smaller particles, bigger hazard
The main pollutant of concern in wildfire smoke is PM2.5—hazardous particles smaller than 2.5 micrometers wide. Their size allows them to lodge themselves deep inside the lungs, causing tissue damage and inflammation. The hexavalent chromium nanoparticles Kleeman and his colleagues detected are an order of magnitude smaller.
“Nanoparticles smaller than 50 nanometers can cross cell membranes, meaning they can get deeper into our bodies than larger particles,” he explained. “Nanoparticles can circulate in our blood and get to all of our major organs.” Still, the specific health risks from hexavalent chromium nanoparticles remain uncertain. “The current findings warrant caution, but not panic,” Kleeman said.
He plans to return to Altadena and the Palisades to determine whether airborne hexavalent chromium levels have returned to normal and identify potential sources and exposure hotspots. Understanding this newly realized threat is more important than ever as global temperatures rise.
“California is in a new reality where climate change is driving wildfires into major urban areas,” Kleeman said. “We all need to work together to adapt to this new reality.”
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