Glowing red skies and bitter ash-laden air are increasingly common across much of the US as the warming climate fuels vast wildfires. For years now, researchers have understood that wildfire smoke, and the noxious gases and soot particles it carries, isn’t merely an unpleasant experience that forces people to shut windows and herd children indoors. It’s a significant health hazard that not only triggers asthma and breathing problems, but can harm immune systems for years.
But new research is revealing that the threat is more complex than previously known—and also more dangerous, if that’s possible. The churning convection in those enormous plumes can scoop up pathogens from disturbed soil and charred vegetation and carry them long distances from the fire lines. Many of those microbes are still living, and potentially capable of causing infections. Among them, researchers are most worried about fungi, which cause slow-growing internal infections that are difficult to diagnose and treat, and can be disabling and even deadly. That includes the fungal infection valley fever, which lurks in dry Western soils, causes a serious illness resembling pneumonia, and is spreading north as the climate warms.
Thanks to drones carrying sampling equipment, there is already evidence that smoke plumes are packed with viable bacteria and fungi. One early study has linked a major 2017 California fire with increasing numbers of fungal infections in hospital patients up to 200 miles away. Now several research projects are attempting to precisely identify the microbial contents of smoke and to correlate infection records with maps of the directions in which smoke drifts during fire seasons.
“We have found ample bacterial and fungal pathogens that do pose a significant risk, particularly to our immunocompromised patients: cancer chemotherapy patients, people on medications that knock down their immune system because of autoimmune disease,” says George R. Thompson, a physician and professor at the University of California, Davis, School of Medicine who specializes in invasive fungal infections. But proving the link to any one patient’s illness can be challenging, he points out: “These smoke plumes often will go over multiple states. If you’re 500 or 600 miles away from a fire, even if you see an increase in a particular infection, a lot of people will not make that association.”
Two years ago, Thompson cowrote a perspective piece in Science that made the case for “bioaerosols” in smoke as a human health hazard. His coauthor Leda Kobziar, a wildland fire ecologist and associate professor at the University of Idaho, builds and flies the drones that dive into smoke plumes to harvest whatever microbial matter they contain, a discipline that she has dubbed “pyroaerobiology.” She has demonstrated that smoke not only contains a dense array of viable bacteria and fungi that can be transported long distances, but also that the types of microbes differ according to the fire’s location, rate of combustion, and fuel—underbrush in a prescribed fire, versus building materials and plastics in one that’s out of control.
“We’re really just at the forefront of understanding this mechanism that has probably been influencing microbial dispersal and diversity of life for as long as we’ve had fires—so, many hundreds of millions of years,” Kobziar says.
Researchers already knew that fires create infectious risks for crews on the front lines of fighting them. In August, a team from the US Centers for Disease Control and Prevention and the California Department of Public Health, among others, reported that seven crew members who battled a 2021 wildfire developed coughs and chest pain and had trouble breathing. Three of them were confirmed by lab tests to have valley fever infections. (Among the other four, two could not be tracked down and two had negative tests—though the CDC says those tests can be inconclusive.)
In 2017, other CDC and California state researchers identified 10 firefighters from a state prison crew who also caught valley fever, some with serious enough cases to develop respiratory failure and meningitis. In both episodes, the firefighters were either enveloped in dust and smoke or doing earthmoving—digging ditches and firebreaks; they told the two sets of investigators that they weren’t given any respiratory protection. So their infections might have come from spores inhaled in smoke or from dust and dirt containing fungi that were kicked up by their digging.
But proximity to dust and dirt probably can’t explain the surge in invasive fungal infections—mold, valley fever, and aspergillosis, from a fungus that thrives in decaying vegetation—that researchers from federal agencies and the University of California, San Francisco documented in everyday people who were admitted to 22 California hospitals following that fire in 2017. It also can’t explain puzzling increases in infections that occur in the same season as fires, according to Naomi Hauser, an infectious disease physician and assistant professor of medicine at UC Davis. “During wildfire season, we get patients admitted to the hospital with unusual infections that they shouldn’t really be at risk of,” she says. They’re especially noticeable in burn patients, whose injuries heighten their vulnerability to whatever infections are blowing through.
Hauser has set up a monitoring project that places air-sampling equipment on the trucks and equipment of regional firefighting departments, and also around the Davis campus and in Sacramento. By comparing the microbes the devices capture, she may be able to narrow down whether similar exposures occur both at the front lines of fires and in cities bathed in smoke—or whether fungal infections in residents arise from local sources instead.
Meanwhile, a $1.2 million project based at the University of Florida, run by a team that includes wildfire expert Kobziar and Jason Smith, a University of Florida associate professor who studies forest fungi, is teasing out the contents of smoke and trying to predict the microbial risks it may pose. In one arm of the study, epidemiologists will map plumes from California wildfires between 2017 and 2020 and overlay them with digitized medical records from the California arm of the Kaiser Permanente health system. (Because it operates both hospitals and clinics—and also has a research division and a medical school—Kaiser is able to collect data about its millions of members in a way that is unusual in the patchwork US health care system. Its comprehensive records of visits, diagnoses, and treatments are a rich resource for researchers.)
About 180,000 Kaiser patients are diagnosed with some form of fungal infection every year, says Stephen Van Den Eeden, a senior epidemiologist in Kaiser’s research division. Most of those infections are likely to be minor everyday skin problems, or the invasive infections expected in severely immunocompromised people. But some of the records may reveal fungal infections that can plausibly be associated with smoke plumes wafting over the places where those patients live. If the layers line up—smoke direction, residence, and diagnosis—that will trigger further inquiries about how far fungal spores might have been transported, and which underlying conditions made people most vulnerable. “Every study leads to 10 others,” Van Den Eeden says. “But the basic thing is, do we see any signal at all that represents an association?”
At a minimum, confirming a fungal hazard from wildfires could lead to recommendations that vulnerable people take extra protective measures on smoky days. But it also could contribute valuable evidence to two separate lines of scientific inquiry. One is the ongoing research into how particulates in smoke—minuscule particles known as PM 2.5, or less than 2.5 microns across—affect the immune system. Particles that small can penetrate deep into airways, and researchers at Stanford University’s Sean N. Parker Center for Allergy and Asthma Research have found that they affect the functioning of the immune system, reducing the production of T cells that help the body defend against allergens and pathogens. It’s possible that fungi could hitchhike into the lungs on those particles, or that immune system impairment gives those fungi a head start when they begin to reproduce.
Fungal research could also enhance wildfire control. Fire managers including Kobziar support using prescribed burns to clear out deadwood and underbrush that would otherwise become a wildfire’s fuel. Prescribed burns don’t just swap one fire (and smoke plume) for another; because they’re set in chosen spots and controlled, they’re fundamentally different fire types. “The conditions under which we ignite fires are more conducive to complete combustion, which means that the pollutant products are minimized in relation to how much fuel is consumed,” Kobziar says. “And the conditions are also prescribed so that the smoke does not remain in highly populated areas for extended amounts of time.”
Prescribed burns are controversial though—and with the West in what seems like a permanent drought, some politicians and resource managers consider them too risky. If it could be shown that they’re less conducive to spreading fungi, that would be a factor in their favor—and potentially make them a tool for knocking back a health threat we’re just beginning to understand.
