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 not only that triggers asthma and breathing problems, but life 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 feverwhich 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 will often 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.