When wildfires rage across the western United States (and other parts of the globe), the destructive power of the fire and plumes of smoke that degrade air quality get most of the attention. Emerging research has added a new concern to that list: Airborne microbes can hitch a ride on smoke and infect firefighters and people living downwind from the fire.
In an article published in Science, researchers Leda Kobziar and George Thompson surveyed existing research and modeled how bacterial and fungal cells can spread through wildfire smoke, concluding smoke plumes can carry dangerous concentrations of pathogens and spread them over wide areas. Smoke-related health risks such as asthma and bronchitis, and even some smoke-related infections, are already well-documented, the researchers point out. Valley Fever, for example, is a fungal infection common among wildland firefighters, and studies show correlations between wildfire events and spikes in bacterial infections in the western U.S. Yet the prevalence of microbes in smoke plumes has received little formal study.
For their assessment, Kobziar and Thompson focused on fungal and bacterial cells, also known as bioaerosols, and examined how they could survive and spread in wildfire smoke. While plumes can become incredibly hot—too hot for any microbe to survive—they also vary greatly based on what’s burning, the behavior of the fire, and how the smoke mixes with the air. Because of that variability, the researchers argue, bioaerosols could be drawn up into the air and survive.
Once in the air, the smoke can be a surprisingly habitable place for those organisms. Carbon is one of the byproducts of fires, and carbon particles in the plume can provide a “temporary habitat for soil microbes,” the researchers write. In addition, the bits of dead plants and particulate matter sucked up into the smoke can protect the bioaerosols from the sun’s UV rays, which would normally destroy them, and water vapor in the smoke keeps them from drying out.
So just how many bacteria and fungal cells could be floating around in wildfire smoke? Kobziar’s estimate is in the trillions.
“At this point, it’s really unknown,” she told the Los Angeles Times. “The diversity of microbes that we’ve found are really mind-bending.”
No previous studies have assessed bioaerosol content in wildfire smoke, but she and Thompson do cite one study that focused on prescribed burns. Even those low-intensity fires produced smoke with microbial counts five times above what’s found in normal air. In their assessment, they modeled how bioaerosols could travel in smoke plumes and found they have the potential to spread hundreds of miles away from the fire itself. Eventually, they write, those organisms get deposited downwind—or inhaled by people.
For Kobziar and Thompson, the assessment is a clear indicator that more research is needed. Climate change is helping wildfire seasons across the globe grow longer and more destructive, and with more smoke in the sky, scientists are concerned the risk of airborne infections may rise.
“We have more questions than answers at this point,” Thompson said.
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