Forest fires leave behind charcoal—and it might be toxic for years

New research shows the compounds created during a blaze stick around, and could be hazardous to humans.

If you stand in the remains of a forest fire in a drizzle, even years after the burn, you can smell woodsmoke rising from the downed logs and charred stumps. The blackened remains might be hiding other things, too.

According to research published Friday in Nature Communications, Earth and Environment, that charred wood contains compounds that have recently been recognized to pose a serious health risk to humans. The environmental implications of those findings are unclear, since wildfires are key to so many ecosystems. But they could be important for understanding the environmental and health consequences of more frequent, intense fires on a warming planet.

The compounds, called environmentally persistent free radicals, are a product of incomplete combustion. Gabriel Sigmund, the study’s lead author and an environmental chemist at the University of Vienna’s Centre for Microbiology and Environmental Systems Science had found the compounds in biochar, a sort of high-tech charcoal used to enhance soil fertility. He realized that if they were showing up there, they might be in naturally burned wood as well.

The radicals aren’t directly dangerous to humans—instead, they hang around, then, in response to a change in the environment (like exposure to water) set off a chain reaction, creating other chemicals called radical oxygen species that can damage cells and DNA. “They may have a very strong effect on the environment, once they begin reacting,” says Slawo Lomnicki, who studies the chemicals at Louisiana State University’s Superfund Research Center.

Lomnicki wasn’t involved in the research, but his team’s work has found that the radicals can contribute to asthma and lung cancer after someone is exposed to soot, smoke, or car exhaust.

Team members from Swansea University in the UK collected samples of charcoal from 10 different wildfires, from boreal pine forest to Mediterranean shrubland to subtropical grasses. The highest concentrations of environmentally persistent radicals were found in the remains of forest fires, especially where trees and other woody plants had burned, and the compounds were found in charcoal collected five years after a fire.

Sigmund says that he expected to find something in the charcoal, but “the fact that we would measure them over five years was surprising.”

In biochar, he says, the radicals tend to break down after hours, maybe a day or two at most, and he hasn’t seen studies on how it ages in the ground. Why the radicals left by wildfire last so long is still something of a mystery. It could be that the chemicals just aren’t reacting with the environment—”they’re just sitting around,” as Sigmund puts it. Or small amounts of certain metal oxides in the soil could be “recharging” the radical compounds as the compounds trade electrons.

But Lomnicki says that the longevity of the chemicals is in line with what he’d expect based on work with particulate pollutants, like engine exhaust. The pieces of charcoal are large enough, he expects, to keep radicals trapped inside where they can’t react with other chemicals.

Of course, fires are—or were, before a century-long fire suppression campaign in the United States—a regular part of many ecosystems. The vast Southern pine forests were once sustained by regular grass fires, and larger blazes regularly swept through California’s Sierras.

“From an ecological or evolutionary point of view,” Sigmund says, it would be surprising if those fire-prone ecosystems weren’t adapted to the radicals, though he cautions that he’s not an ecologist or evolutionary biologist. “I could imagine that in many, many cases, this is a facet that we were not aware of, but that doesn’t mean the ecosystem was not aware of it.”

But as the planet warms, fires are appearing in new places. Sigmund points to recent blazes in the Arctic tundra and the Amazon, where it’s less likely that plants, animals, and microbes are adapted to the aftereffects of fire.

The growing intensity of fires also changes their role in a landscape. The mega and gigafires of California have burned through forest canopy, leaving huge stands of dead trees. Sigmund says that those crown fires are very similar to the conditions in their study that produced more radicals: crown fires in Australian eucalyptus and the Canadian boreal forest.

“We usually have a higher temperature of the fire for a prolonged period of time,” he says. “Those are conditions that form higher concentrations and more possibly more persistent free radicals.”

This research doesn’t, however, hint at where the radicals go from the wood.

“The really interesting open question is, what happens after fire? What I believe could have the highest impact is surface runoff into rivers,” Sigmund says. “You’d have a lot of material that would reach the system at once, just a spout of stuff.” There’s a large body of evidence showing that streams and rivers run higher, and with more sediment, after fires, because there are no more plants to hold water and curb erosion. There’s also evidence that those post-fire waterways contain different species of insect and fish, some of which seem to thrive or hate the high-runoff aftermath.

All that could send charcoal into waterways, where it could react with water to form the potent oxygen radicals. Lomnicki says that he can only speculate on the impacts of that, since research on environmentally persistent radicals is still so new. In theory, he says, “if you have a small organism, these radicals can damage the membranes, the cell walls, and this can cause leakage.”

However, in the wake of fires, lots of new chemicals, minerals, and organic material is introduced into waterways, making it hard to pin down what exactly changes the ecosystem. And some research suggests that charcoal and wood ash might spur growth of microorganisms in the soil, at least in fire-adapted ecosystems.

But, Lomnicki says, the compounds are already showing up in wildfire smoke. Public health experts have long warned that smoke can lead to heart problems and inflammation, but the ways it causes damage are complicated and not completely settled. But based on Lomnicki’s research, environmentally persistent free radicals are likely part of that story. “Smoke is essentially tiny particles suspended in the air. Over time, when the smoke diffuses, you don’t see it anymore. But that doesn’t mean it isn’t there.”