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Is there a link between wildfires and infectious diseases?

Fueled by new evidence that wildfire smoke can carry living microbes and is associated with a local increase in COVID-19 cases, scientists wonder if smoke might spread infectious diseases and worsen their effects

by Benjamin Plackett, special to C&EN
December 6, 2021 | A version of this story appeared in Volume 99, Issue 45


A photograph taken during the California "River Fire" of Salinas, in Monterey County. The photos shows a fire burning near a home.
Credit: David A Litman/Shutterstock
A wildfire burns near a home in Salinas, California.

Wildfires have become increasingly common in many parts of the world—including in Australia, the Middle East, and the US—forcing people, sometimes whole towns, to flee the approaching flames. In addition to the immediate damage posed by these massive fires, public health experts have known for decades that the particulate matter in wildfire smoke poses a significant risk to respiratory and cardiovascular health.

Recent studies have some scientists wondering if the smoke might exacerbate or even transmit infectious diseases. Researchers want to follow up on these studies to better understand wildfire smoke’s potential risk to public health as major fires become more frequent and severe.

Leda N. Kobziar is one of those researchers. She is a wildland fire scientist at the University of Idaho, working in the burgeoning field of pyroaerobiology: the study of viable microbes aerosolized within wildfire smoke. Until 3 years ago, scientists assumed smoke was a sterile substance, a potent mix of harmful gases too hot to sustain thriving microorganisms. A flurry of recent experiments has suggested the opposite, however, with data indicating that wildfire smoke can be a source of living bacteria and fungi.

In 2018, Kobziar and colleagues reported on a study in which they exposed Petri dishes to wildfire smoke for 2 min at varying distances from the fire’s epicenter and then observed whether any microbes grew on the dishes. The researchers could culture 70 types of fungi and bacteria. The abundance of culturable microbes fell with greater distance from the blaze, indicating that the organisms came from the smoke (Ecosphere 2018, DOI: 10.1002/ecs2.2507). In an unpublished study, currently being peer-reviewed, Kobziar found human pathogens such as the fungi Aspergillus penicillioides present in smoke.

I would expect to find no microbes in the core of the fire as it is too hot.
Katharina Richter, a microbiologist at the University of Adelaide

“I’ve since become kind of hooked on the idea because I find it so fascinating,” Kobziar says. It’s theoretically possible that viruses could spread in the same way, she says, but her team hasn’t investigated them yet. Because viruses can’t be cultured on Petri dishes the way bacteria or fungi can, scientists would need to search for fragments of viral DNA using a polymerase chain reaction test.

For now, though, the revelation that wildfire smoke contains life is a surprise to scientists. “I would expect to find no microbes in the core of the fire as it is too hot,” says Katharina Richter, a microbiologist at the University of Adelaide who was not involved with the study. “The viability and composition of microbes transported by smoke may have significant implications for forest health,” in addition to public health, she says, because some of the microbes could be plant pathogens.

Kobziar doesn’t exactly know how microbes can survive in smoke plumes, but she has done additional research on the heat tolerance of soil-dwelling microbes to show that some can survive at 800 °C. Wildfire flames burn at temperatures ranging from 300˚ to 1,100 °C.

Off the back of these studies, Kobziar and George Thompson, a professor of clinical medicine at the University of California, Davis, who specializes in fungal infections, have proposed a theory on how wildfire smoke may spread microbes. When forests and plants burn, the soil becomes agitated, churning out fungi and bacteria that would usually reside peacefully within it. “Hot fire creates its own weather, so there are a lot of updrafts, which toss microbes into the air,” Thompson says. In this whirlwind, microbes collide with and stick to ash and smoke particles of varying sizes that can travel long distances. A recent study reported that Australia’s wildfires in 2020 created such a large, heat-fueled vortex that smoke reached an altitude of 35 km (Commun. Earth Environ. 2020, DOI: 10.1038/s43247-020-00022-5). Another paper showed that smoke from the same Australian fires traversed the Pacific Ocean to reach Chile (Atmospheric Chem. Phys. 2020, DOI: 10.5194/acp-20-8003-2020).

“Can microbes stay viable across oceans?” Kobziar asks. “It’s hard to make specific conclusions, but we know they can survive on dust particles for a long time, so it’s reasonable to think it’s possible, but this isn’t something we’ve tested yet.”

Scientists also aren’t yet sure whether concentrations of specific microbes in smoke are sufficient to present a direct infection risk for people. Kobziar is trying to better understand the risks of microbes in wildfire smoke and is now studying fresh smoke samples taken closer to a fire’s source. “We decided to take it up a notch and gain access to the bigger and more extreme fires,” she says. To achieve that, she and her colleagues rely on drones fitted with vacuum pumps to draw microbes through a filter, where they’re captured for analysis. This system should allow the team to learn how the microbe profile differs at the epicenter of a wildfire and further understand how microbe abundance decays with distance.

A graduate student collects samples of wildfire smoke to analyze its microbial content.
Credit: Leda N. Kobziar
University of Idaho graduate student Phinehas Lampman collects air samples from the Caldor Fire in California in August 2021.

While scientists like Kobziar learn more about whether wildfire smoke can spread pathogens, others think the fires make some existing infections worse. Earlier this year, researchers reported that people located in areas with wildfires tend to have more severe COVID-19 symptoms than those in unaffected precincts.

In 2020, the first year of the COVID-19 pandemic, over 10 million acres of the western US burned, with both California and Washington posting their worst fire seasons on record. Kevin Josey, a biostatistician and postdoctoral research fellow at Harvard University, estimated the number of COVID-19 cases and deaths associated with the 2020 wildfires (Science 2021, DOI: 10.1126/sciadv.abi8789). To do so, Josey and colleagues created a statistical model considering several parameters, including levels of wildfire particulate matter, how often people were moving around outside their homes, and whether the pandemic itself was broadly in decline or on the rise, to estimate COVID-19 cases and deaths. The researchers used publicly available data on daily concentrations of wildfire particulate matter from 92 western US counties and showed that their model could estimate COVID-19 case and death numbers from those counties.

Running the same model but using average particulate matter readings from 2016 to 2019, the team predicted what would have been the COVID-19 case and death rates on “normal” days, without wildfire smoke. Their results suggest that the 2020 wildfires increased cases and deaths by 2.9% and 1.02%, respectively.

Josey thinks a connection between wildfire particulate matter and worse COVID-19 outcomes would be logical. “Your lungs become more inflamed when exposed to smoke, making it more difficult to breathe,” Josey says. “COVID-19 is a respiratory disease, so it makes sense that that smoke would cause worse outcomes.”

Other researchers have also found evidence for a potential link between smoke and COVID-19. Daniel Kiser, a data scientist at the Desert Research Institute, looked at whether the 2020 wildfire smoke in Reno, Nevada, was associated with a bump in the area’s COVID-19 case rate. He found that a 200% rise in the weekly average concentration of particulate matter coincided with a 6.3% increase in the average weekly case rate (J. Expo. Sci. Environ. Epidemiol. 2021, DOI:10.1038/s41370-021-00366-w).

These studies build on previous findings that wildfire smoke may have a negative effect on respiratory infections. A 2013 paper reported that such fires overlap with significant upticks in the number of people seeking medical attention for respiratory illnesses, and a 2020 study reported a similar pattern with influenza.

The reasoning sounds plausible, according to Najat Saliba, an atmospheric chemist at the American University of Beirut. “We know pollution causes respiratory problems, and once COVID-19 comes with these problems then it’s probably going to be exacerbated,” she says. But Saliba warns that the science on this isn’t definitive yet.

Saliba notes that scientists spent decades collecting data to establish the link between particulate matter smaller than 2.5 µm and increased mortality and that the same type of effort will be needed to make the connection with infectious disease. “The causality between COVID-19 and wildfire smoke hasn’t been proven yet,” she says.

Josey admits that this is fair criticism. Despite his team’s best efforts to control for non-wildfire variables that could have led to a spike in COVID-19 cases, he says the group couldn’t account for everything. For example, Josey later found out that some of the Washington State wildfires were concurrent with students’ returning to campuses. Such events make it hard to tease out the effect of wildfire pollution on COVID-19 severity versus the effect of extra social mixing that likely occurred.

Kiser notes similar issues with excluding non-wildfire variables from his team’s analysis. “When air quality is bad, people may be more inclined to socialize indoors, so human behavior could be playing a role here,” he says. Both Josey and Kiser plan further study on a connection between wildfire smoke and COVID-19.

As wildfires become more frequent with climate change, fully understanding how they interact with infectious diseases is going to become critically important, according to Kobziar. “It’s such a deep unknown that it’s worthy of effort,” she says, “and it could help us understand patterns of human illness in the long term.”

Benjamin Plackett is a science journalist based in Australia.


This story was updated on Dec. 8, 2021, to correct where the author is based. Benjamin Plackett is based in Australia, not London.


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