In a new study published on Friday, a team of researchers at Harvard University found evidence that exposure to elevated levels of fine particle pollution found in wildfire smoke may have led to thousands more cases of COVID-19 and more deaths for those who tested positive for coronavirus.

In some counties in California and Washington state hit particularly hard by wildfires last year, the study, published in the journal Science Advances, concluded that nearly 20% of the COVID-19 cases were attributable to elevated levels of wildfire smoke. They also found an even higher percentage of deaths could be linked to wildfire smoke in certain counties.

“Clearly, we see that overall this is a very dangerous combination,” Francesca Dominici, a professor of biostatistics at Harvard and one of the authors of the study, said of the interplay between smoke and COVID-19. “It’s a really scary thing as we continue to face these wildfires all around the world.”

For people living in parts of the American West, the pandemic was just one of the disasters they faced last year.

In 2020, California saw five of the six largest wildfires in the state’s recent history, a record cluster of mega-fires that the currently burning Dixie Fire has since elbowed its way into, reaching number two on the largest fires list. Smoke from these and dozens of other fires blanketed Western states last year, and huge plumes powered record-breaking “smoke storms” that choked Oregon and Washington.

Earlier studies have found evidence that air pollution can worsen the severity of COVID-19 symptoms and hasten the spread of the virus, although there is still uncertainty regarding how particles and the virus interact in the air.

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On its website, the U.S. Centers for Disease Control and Prevention says that wildfire smoke can irritate lungs and “make you more prone to lung infections, including SARS-CoV-2, the virus that causes COVID-19.”

A study published earlier this year by scientists at the Desert Research Institute in Nevada found that COVID-19 positivity rate in Washoe County increased significantly during periods of high wildfire smoke. The Harvard study sought to quantify the relationship across a wide range of counties in the American West.

To investigate the connection between smoke and COVID-19, the researchers — from the Harvard T.H. Chan School of Public Health and the Harvard School of Engineering and Applied Sciences — developed statistical models that analyzed data from 92 counties in California, Oregon, and Washington where wildfires occurred between March and December of last year. In those three states, more than 73,000 people have died of COVID-19 during the pandemic.

The researchers gathered information on COVID-19 cases and deaths in those counties over time. They used satellite sensors to track when wildfires were burning and ground-level sensors to follow levels of fine particulate matter, known as PM2.5, a major pollutant in wildfire smoke. They included a monthlong lag to allow for health impacts to develop after infection. Their models also took into account other variables such as weather, population and general trends in the pandemic to control for other factors that might skew the results, Dominici said.

The study found “strong evidence” of a link between increases in particulate matter and risks of COVID-19 cases and deaths.

“We found that, in some of the counties, the percentage of the total number of COVID-19 cases and deaths attributable to the high levels of PM2.5 was substantial,” the study said.

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The counties of Butte, Calif., and Whitman, Wash., showed some of the largest impact of wildfire smoke on COVID-19 cases: The study’s models found that 18.2% of Whitman’s COVID-19 cases last year, and 17.3% of Butte’s cases, could be attributed to poor air quality on wildfire days.

As wildfires have burned huge swaths of the West in recent years, public health authorities have increasingly been confronted with ailments associated with ingesting heavy doses of smoke. Authorities say smoke can exacerbate respiratory conditions such as asthma and chronic obstructive pulmonary disease. The mental health consequences, including anxiety and depression, that come with living under a blanket of smoke can also be profound.

In other research last year, “the evidence started to show that [smoke] could lead to worse health impacts” for COVID-19 patients, said Kaitlyn Kelly, air quality policy specialist at the Washington State Department of Health. “If you have COVID-19, breathing in wildfire smoke may make your symptoms worse. Or wildfire smoke can you make you more susceptible to respiratory infections.”

Kelly said it was good to see research coming out such as this Harvard study that is “getting to confirm what we started to see last season.”

Scientists who study air quality say that it is possible that smoke particles could in effect carry the virus. There are also other possible dynamics involved, such people tending to gather indoors to avoid wildfire smoke, which could lead to more interaction with other infected people.

“When there are more particles in the air, these microbes actually have a greater chance of getting into your lungs,” said Irva Hertz-Picciotto an environmental epidemiologist at the University of California, Davis, who studies the health impacts of wildfire smoke. “There’s a lot of plausibility that the wildfires, by massively increasing the amount of PM2.5 that people are breathing, could promote transmission of the virus.”

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Hertz-Picciotto, who was not involved in the Harvard study, said that previous studies have often looked at emergency room visits and hospitalizations for extreme respiratory conditions. She has been involved in a longer-term study tracking health impacts on people in California affected by wildfires since 2017.

“Given that these are going to be with us year after year, we have to rethink a lot of things about our safety nets and our society as a whole,” she said. “What kind of health resources do we have, and how can people protect themselves?”

The same Harvard team also published a study last year linking exposure to air pollution with COVID-19 death rates. The scientists involved have worked on air pollution epidemiology for many years.

“These are hard data questions but it’s also a group with a lot of experience working on these kind of questions,” said Lance Waller, a professor of biostatistics at Emory University who was not involved in the study. “They’re making the point that the air is unhealthy to breath, and then if you’re in a compromised situation because you’re ill with COVID-19 and having respiratory problems anyway … it’s a double whammy.”

Beyond increasing number of coronavirus cases, the Harvard study also found that health outcomes from COVID-19 worsened as air quality deteriorated. An increase of fine particulate matter of 10 micrograms per cubic meter was associated with an increase of 8.4% of COVID-19 deaths across all counties. The U.S. Environmental Protection Agency characterizes air quality as “unhealthy” in the range of about 55 to 150 micrograms per cubic meter.

In Butte, for example, the researchers attributed 41% of the COVID-19 deaths to high levels of wildfire smoke. There have been about 14,000 COVID-19 cases and 200 deaths in Butte County during the pandemic, according to data compiled by The Washington Post.

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To reach these estimates, the researchers analyzed deaths in previous years in those same counties on days when wildfire smoke wasn’t present, Dominici said.

Dominici said her team’s results underscored both the importance of vaccinations but also that people should do as much as they can to avoid dangerous levels of wildfire smoke.

“Especially in these counties I think that absolutely everybody should get vaccinated ASAP because this study points [out] that these counties can be more affected both in terms of cases and deaths,” she said.

She added: “I understand it’s not feasible for everyone, but to the degree they can: Move and be away from this exposure to wildfires at this time we have this delta variant coming in.”