For two long months in the summer of 2018, we in Durango, Colorado woke each morning to a thick blanket of smoke hanging over our homes. Water-starved vegetation and stray embers from a coal-fired train had combined to ignite the 416 Fire, sending our normally active community indoors to escape the smoke. Many here worried about the smoke’s impact on our lungs, but few, if any, of us realized that there was a hidden threat that came with the haze: an increased risk of respiratory infections.

The 416 fire was part of a brutal wildfire season in Colorado, and the American West more broadly. It was soon surpassed by the extreme 2020 wildfire season, which set records across the West. Like many areas globally, the region is experiencing increasing temperatures, stronger winds, and shifts in precipitation, contributing to more frequent and intense wildfires and a longer fire season.

Of course, 2020 also brought the Covid-19 pandemic. And as more than twenty fires burned in California that fall, turning the sky an opaque orange over San Francisco, scientists there identified a concerning connection: Higher particulate pollution and carbon monoxide levels correlated with higher Covid case rates and deaths in the city. Expanding upon this finding, a larger 2021 study revealed a similar link between levels of particulate air pollution and the risk of contracting and dying from Covid across California, Oregon, and Washington during the 2020 fire season. Prior to these studies, the evidence for a link between wildfires and respiratory infections was mixed, according to a 2019 research review by Colleen Reid, an associate professor of geology at the University of Colorado Boulder.

Kevin Josey, first author of the 2021 study, had been at the time a recently-hired postdoc in the lab of biostatistician and data scientist Francesca Dominici at Harvard. But the Covid lockdown kept him where he was in Colorado. Phlegmy lungs from walks during Colorado’s fire season inspired his involvement in the study. “During that lockdown…a bunch of wildfires started happening,” he comments. Dominici had already been pondering how wildfire smoke might be impacting human health, and, given the concurrent pandemic, the thought became “we may as well look at what the combined effect might be of these two things,” says Josey.

Josey – now an assistant professor of biostatistics at the Colorado School of Public Health – notes that while overall air pollution has decreased in the US since passage of the Clean Air Act (CAA) in 2000, increased wildfire smoke has undermined many of those gains in recent years. And that’s not good for public health. Particulate pollution exposure itself has been link to increased risk of respiratory problems, heart attack, lung cancer, stroke, and declines in cognitive function. And beyond Covid, scientists have linked smoke exposure to increased incidence of influenza, tuberculosis, and other respiratory infections. As climate change leads to more frequent and severe wildfires, these trends may worsen.

No one knows precisely what’s behind the link between smoke exposure and respiratory infections. Writing about Covid, Kathleen Navarro and co-authors hypothesize that tiny particles in smoke transport the causative virus, SARS-CoV-2, deep into the respiratory tract, where the smoke also renders airway cells more permissive to infection. This reflects evidence that SARS-CoV-2 sticks to airborne particulates and that smoke exposure increases the number of receptors (“entryways”) that the virus employs to infect cells. A higher level of infection, together with airway damage caused by both smoke and virus, may translate to more severe Covid. Weakened immunity and airway damage from smoke exposure may also leave people more susceptible to infection.

Navarro coordinates the Wildland Firefighter Health and Wellbeing Program for the Department of the Interior. She’s familiar with working in smoky conditions. “I was on a lot of fires,” she says, having spent a season on a hotshot crew and years interning with the Forest Service, filling in on fire crews, and working with the National Technology and Development Program to study smoke exposure on site. Her on-the-ground experience informs her work in developing approaches to protect firefighter health: “If we’re going to suggest recommendations, it has to be rooted in the reality of what the job is.”

Another recent study, published last year, builds on the findings of Navarro and her collaborators. Looking at the H1N1 pandemic strain of influenza virus, the researchers found that tiny particles from smoke and other pollution sources could bind the viruses and carry them deep into the lungs of mice. There, the particles enhanced infection in multiple ways and facilitated viral spread to organs beyond the lungs, resulting in more severe symptoms than those from particles or virus alone.

From an epidemiological standpoint, sussing out the combined health risks of wildfire smoke or dust inhalation and respiratory pathogens is complicated. Then there are the sociodemographic factors to consider. That’s where the aforementioned Reid focuses her work, studying how wildfire smoke exposure and sociodemographic factors interact to impact health. Both her and Josey’s research reveal disparities in health outcomes from air pollution according to people’s socioeconomic position. Reid notes, for example, that “people live in vastly unequal homes” and therefore have different levels of smoke exposure. Similarly, research shows that those in California with less access to housing and healthcare suffered the most from the convergence of Covid and the 2020 wildfire season.

Reid sees such research as having a role in influencing climate policy, but feels that it’s not happening quickly enough. “I’m moving into how we can protect people’s health while waiting for the powers that be to move the needle on [climate] action,” she says.

So are others. For instance, Navarro’s squad leader shifted her hotshot crew out of the heavy smoke from a burn along the Deschutes to work in less smoky locations. Concerning the general public, Reid points out that the current approach is to encourage mask and air filter usage during smoke events. When used properly, both also reduce the risk of Covid. But she notes that people’s differing living and job situations create significant impediments to enacting such recommendations. Masks aren’t practical for firefighters digging a fire break, which is physically arduous, and air purifiers aren’t universally affordable. Still, there are ways to address some of these barriers. For example, Reid points to a Missoula, Montana program that loans out air purifiers during smoke events. The program prioritizes who gets them based on factors such as existing health conditions and economic status.

“There are things that can be done in a creative way,” Reid says.