How wildfires can spread cancer-causing chemicals

Wildfire smoke is known for producing and spreading high levels of harmful fine particulates, but it may also be creating carcinogenic soil and ash, according to new research.

The study, published Tuesday in Nature Communications, found that California wildfires activate a chemical element called chromium in its carcinogenic form in soil and create heaps of cancerous ash from burned infrastructure and vegetation. Layers of soil become carcinogenic because of high wildfire temperatures – and all the ash that forms and builds up could linger for nearly a year after the flames die down.

It has long been known that when buildings, vehicles and trees burn, high levels of metals end up in soils and ash. But this study takes a closer look at the harmful effects of chromium that are naturally occurring, and normally benign, once they encounter blazes and their associated high temperatures.

In the study, researchers examined the toxic chemical makeup of fine particulate matter, also known as PM2.5, showing that the chemical makeup can be cancerous because of the chemistry and not just size, said co-lead study author Scott Fendorf, a professor at Stanford University’s Doerr School of Sustainability.

“Beyond just the physical toxicity, there’s a chemical toxicity to these particles that, depending on where the fire occurs, can be quite hazardous,” Fendorf said.

Exposure to the carcinogenic form of that chemical element, called hexavalent chromium, during and after wildfires can increase risks of lung, sinus and nasal cavity cancers, the study said. Hexavalent chromium is a known human carcinogen when inhaled and has been shown to cause tumors in mice and rats when ingested through drinking water, according to the Environmental Protection Agency.

While the study raises concerns about the distributions of hexavalent chromium in soil and ash, there is still a lot to research about its airborne and water contamination impacts on humans, said Joel Kaufman, a professor in the University of Washington’s department of environmental and occupational health sciences who was not involved in the study.

“They are showing us that the ash can turn into a more toxic kind of ash. However, that doesn’t mean that this is what’s in the air that people are breathing in with the smoke,” Kaufman said.

The study implies that there could be impacts because of the abundance of hexavalent chromium but did not directly demonstrate how its production impacts human health during, and after, wildfires.

“We’re still several steps away from being able to show that fire-derived airborne hexavalent chromium is present and causing health effects,” Kaufman added.

In the study, researchers found that levels of toxic chromium increase based on the longevity of fires, location and the substances that the fire is burning. Data showed that soil particulates in severely burned areas in California had harmful hexavalent chromium levels nearly seven times higher than unburned areas.

Metal-rich geologies such as the western United States, Brazil, Australia, South Africa, Europe and Indonesia could supply a hardy amount of chromium, because of the high levels of metal in the soil, depending on fire severity. The more severe the fire is, the higher concentrations of hexavalent chromium, Fendorf said.

“The more chromium you start with in the soil and plants and the greater the severity of the fire, the more hexavalent chromium you get,” Fendorf said.

Urbanized dwellings tend to have more materials with chromium in them, such as stainless steel and chrome plating, which could lend itself to more hexavalent chromium production during wildfires.

Fire severity also determines how much carcinogenic ash was found. In moderate fire severity sites, levels of hexavalent chromium increased nearly five times higher than average concentrations at some spots, which exceeded the EPA’s recommended screening level for carcinogens in residential soils.

As climate change exacerbates extreme droughts in some regions and impacts rainfall averages, hexavalent chromium levels could become a menacing feature in future wildfires.

In places such as California, which experienced three straight years of drought until summer 2023, the dry brush routinely create kindling for wildfires. The state also has many regions with high metal concentrations in the soil.

As wildfires become more severe and prevalent, the particles in them could cause air quality and environmental conditions to be more toxic. Hexavalent chromium could linger for longer without major rainfalls.

“As we’re experiencing more wildfire impacts, largely as a result of presumably climate change and food source management practices, the health and ecosystem impacts are going to be more complicated than we realize yet,” Kaufman said.

“There may well be other chemical and downstream effects that are happening that we haven’t even figured out yet,” he added.

Fendorf hopes the knowledge of chemical toxicity will help with future risk analysis and mitigation efforts.

One step would be to wear N95 masks, he said.

“The first step is that we all start understanding the risk of breathing wildfire smoke much more thoroughly, including the first responders,” he said.