In summer 2017, the California Department of Public Health (CDPH) was notified of a mysterious illness that had afflicted 10 inmate wildland firefighters. After several days at the frontlines of a wildfire, the firefighters had abruptly developed symptoms ranging from fever to malaise to cough; five went on to develop pneumonia, one developed meningitis, and one required intubation for respiratory failure.
In the ensuing investigation, the patients were diagnosed with coccidioidomycosis, a fungal infection known colloquially as “Valley Fever.” Valley Fever was historically confined to California’s Central Valley and Coast, but cases had skyrocketed in the years leading up to the outbreak. CDPH concluded that the firefighters had likely become infected during dust-generating activities, such as digging fire lines or extinguishing hot spots.
Today, the association between wildland firefighting, Valley Fever, and dust exposure has become something of a truism. More recently, research groups have begun asking another question: does exposure to wildfire smoke also impact the likelihood of acquiring a respiratory infection?
The association between ambient air pollution and negative health effects has been recognized for a half-century. In 1970, the Clean Air Act was created to give the Environmental Protection Agency (EPA) authority to monitor levels of air pollutants understood to be harmful to humans. Of these, one of the most hazardous — and ubiquitous — is fine particulate matter (PM2.5), a mix of tiny liquid and solid particles less than 2.5 microns wide, smaller in diameter than a human hair.
PM2.5 is capable of both carrying toxic chemicals and reaching deep into the un-ciliated, gas-exchange portions of the human respiratory tract, where it wreaks havoc. Ambient PM2.5 exposure has been linked to cardiovascular complications, asthma and COPD exacerbations, and respiratory infections. The World Health Organization estimated that in 2019, exposure to PM2.5 was directly responsible for 4.2 million premature deaths globally.
For a long time, it was assumed that wildfire smoke affected human health in much the same way as ambient air pollution. But as the annual incidence of wildfires has grown exponentially, we’re learning that there are nuances to lung injury due to wildfire smoke compared to other types of pollution.
Not All Smoke Is Created Equal
To begin with, the composition of wildfire smoke is highly variable, depending on factors such as what’s burning and ambient weather and atmospheric conditions. Fires at the wildland-urban interface often consume both natural vegetation and human-made materials such as vinyl, plastic, furniture, glass, vehicles, and electronics. The resulting smoke is an especially toxic mix of PM2.5 and an alphabet soup of chemicals, including volatile organic compounds, polycyclic aromatic hydrocarbons, hydrogen cyanide, carbon monoxide, lead, arsenic, and mercury.
In one study, wildfire-specific PM2.5 was calculated to be 10 times more toxic than ambient PM2.5, based on a review of hospital respiratory admissions in Southern California. In another study, wildfire-specific PM2.5 was found to be 10 times more harmful to children’s respiratory health, as measured by urgent care and emergency department visits.
Furthermore, the cadence of wildfire smoke is different from air pollution. Wildfires are associated with sudden PM2.5 spikes that often exceed the EPA’s recommended threshold for days or weeks at a time; in some wildfires, PM2.5 has topped 400 µg/m3 (the recommended threshold is 35 µg/m3 in a 24-hour period).
Wildfire smoke is also capable of impressive drift, traveling hundreds or even thousands of miles downwind. As it travels, it evolves. Sometimes the concentration of pollutants in the smoke decreases. At other times, new pollutants such as ozone or secondary PM2.5 form through chemical oxidation, and the smoke’s toxic potential actually increases downwind.
Finally, it appears that wildfire smoke is capable of transporting bioaerosols — airborne fungal and bacterial cells. It was previously assumed that the high heat of a wildfire would incinerate any microbes. But the nascent field of pyro-aerobiology has shown that the degree of heat within wildfire smoke is highly variable, and that microbes can survive in pockets of particulate matter and water vapor. These microbes essentially hitchhike within the smoke’s PM2.5 to be deposited or inhaled downwind.
The Curious Connection Between Wildfire Smoke and Respiratory Infection
How is this impacting our health? Increasingly, it appears that where there’s wildfire smoke, there’s also respiratory infection. In 2020, California, Washington, and Oregon experienced a series of wildfires of unprecedented intensity. When a group of researchers from Harvard examined the intersection of these wildfire days and COVID-19 infection, they found an 11.7% increase in COVID-19 cases and an 8.4% increase in COVID-19 deaths when daily PM2.5 increased 10 µg/m3 for 28 days. In some counties, this rose to a whopping 65% increase in both COVID-19 cases (Santa Rosa, California) and mortality (San Bernardino, California).
Around the same time, investigators from the University of Montana explored a potential connection between wildfire smoke and influenza infections. They found that between 2009-2018, higher PM2.5 levels during wildfire season were associated with increased influenza cases months later during flu season. For every 1 µg/m3 increase in daily PM2.5 exposure during wildfire season, influenza cases increased by 16-22%.
In 2023, researchers in San Francisco investigated the link between wildfire smoke and systemic fungal infections. In a retrospective analysis of 22 hospitals across California, the authors used smoke plume data to calculate wildfire smoke exposure and its impact on coccidioidomycosis infections. They found that between 2014-2018 hospital admissions for Valley Fever increased 20% in the month following any smoke exposure compared to months with no smoke exposure (or 2% per day of smoke exposure in the prior month). Apparently, firefighters are not the only ones at increased risk of contracting Valley Fever during wildfire season.
This curious connection between wildfire smoke and respiratory infection is becoming ever more relevant as the incidence and smoke intensity of wildfires continues to grow. For four decades, standards established by the Clean Air Act forced a steady drop in ambient PM2.5 levels. But approximately 15 years ago, this trend began stagnating and then reversing. Today, wildfire smoke is responsible for 25% of the country’s PM2.5 levels, and up to half the PM2.5 in some Western states. A recent analysis found that wildfire smoke has reversed a quarter of the air quality gains achieved by the Clean Air Act since 2000.
Toward Greater Understanding
Many questions remain. What is the mechanism by which wildfire smoke makes people susceptible to respiratory infection? Who is most at risk, and are there inequities in who is experiencing these infections? Can the risk be mitigated with personal protective equipment?
Given the relentless projected trajectory of wildfires — a phenomenon attributed to climate change — it seems crucial that our understanding of the relationship between wildfire smoke and human health continues to evolve. Perhaps we’ll find that those bioaerosols are leading to shifts in the endemicity of certain infections, like coccidioidomycosis. Perhaps we’ll tease out the effects of acute versus chronic exposure, given that certain populations are now experiencing regular and repeated exposure to wildfire smoke. For now, we’re only just beginning to understand what wildfire smoke is capable of. And the clock is ticking.
Mary Meyer MD, MPH, is an emergency physician with The Permanente Medical Group. She also holds a Master of Public Health and certificates in Global Health and Climate Medicine. Meyer previously served as a director of disaster preparedness for a large healthcare system. Laura Myers, MD, MPH, is a practicing pulmonologist and intensivist with The Permanente Medical Group. She is a research scientist at the Kaiser Permanente Division of Research and is funded by the NIH on a project to protect individuals with chronic lung disease from the health harms of wildfire smoke. This perspective is the authors’ alone and does not necessarily reflect that of any institutions or companies with which they are affiliated.
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