Wildfire Smoke May Raise COVID-19 Risk, Study Says

Five months into the coronavirus pandemic, a British Columbia Centre for Disease Control researcher issued a warning for the parts of the world that are regularly seeing more extreme and frequent wildfires. 

“As we enter the wildfire season in the northern hemisphere, the potential for a dangerous interaction between SARS-CoV-2 and smoke pollution should be recognized and acknowledged,” Dr. Sarah B. Henderson wrote in the American Journal of Public Health at the time.

Now, a new study provides evidence that bolsters Henderson’s prediction. The research, published in the Journal of Exposure Science & Environmental Epidemiology July 13, found that the number of COVID-19 cases in Reno, Nevada increased by nearly 18% during the period in the summer and fall of 2020 when the city was most exposed to smoke from nearby wildfires. 

“Wildfire smoke may have greatly increased the number of COVID-19 cases in Reno,” the study authors concluded. 

Particulate Matter and COVID-19

The reason scientists were worried about the relationship between wildfire smoke and COVID-19 cases is that there was already a growing body of evidence that air pollution generally—in particular the kind of air pollution known as particulate matter (PM) 2.5—makes people more susceptible to respiratory infections. Even before the current pandemic, researchers found an association between air pollution exposure and the risk of mortality from SARS (or SARS-Cov-1) in 2005. A review of the evidence published in December 2020 concluded there was a strong case to be made that PM2.5 and nitrogen dioxide pollution was contributing to the spread and the deadliness of the new coronavirus as well. 

There are three main theories as to why air pollution makes people more susceptible to respiratory infections like COVID-19, lead author of the Reno study and Desert Research Institute scientist Daniel Kiser explains to Treehugger. 

1. Particulate matter exposure can weaken the lungs' immune response.
2. Microbes, including COVID-19, can hitch a ride on air pollution particles.
3. For COVID-19 specifically, there is evidence that exposure to PM2.5 and nitrogen dioxide can increase the expression of the ACE2 receptor in respiratory cells, which is the molecule that COVID-19 binds to. 

Wildfire smoke presents a concern in this context because it is a major source of PM2.5 that can last in an area from days to months, as Henderson pointed out in her letter. There are differences between wildfire smoke and regular urban air pollution, Kiser says, but there is not enough evidence yet to determine if the composition of smoke makes it more likely to spread disease than other particulate matter sources. However, there are worries associated with the amount of pollutants smoke contains.

“PM2.5 levels from wildfires can be a lot higher than urban air pollution,” Kiser says, “so that could make it more of an issue.” 

Reno 9-11

To find out if wildfire smoke was really increasing COVID-19 risk, Kiser and his research team looked at what happened in Reno, Nevada during an unprecedented summer.

“During the second half of the summer of 2020, two crises converged on residents of the western United States: the second wave of the COVID-19 pandemic and widespread wildfires,” the study authors wrote. “As a result of the wildfires, many residents had prolonged exposure to smoke containing elevated levels of particulate matter 2.5 µm in diameter or smaller (PM2.5).”

The researchers, therefore, looked at particulate matter levels and positive COVID-19 tests in Reno for the period from May 15 to Oct. 20 of last year. For air pollution, they relied on readings from four air quality monitors in Reno and Sparks as publicized by the Environmental Protection Agency. For COVID-19 tests results and patient demographic information, they used data provided by Reno’s Renown Health network. Comparing the data led to two main results suggesting a link between smoke exposure and COVID-19 infection.

1. For every 10 micrograms per cubic meter increase in weekly PM2.5 concentrations, the rate of positive tests went up by 6.3%. 
2. Positive test results increased by around 17.7% from Aug. 16 to Oct. 10, when Reno was most impacted by wildfire smoke.

Kiser acknowledges that the study proves correlation only, and not causation. It is possible that the smoke and positive tests only increased in tandem by coincidence, or that they were more indirectly connected. For example, the smoke could have prompted behavioral changes that encouraged disease spread. 

“People could be spending more time indoors with other people because they don't want to be outside in the wildfire smoke,” Kiser says. 

However, there are a few factors that suggest a casual relationship. For one thing, Kiser says the researchers found that smoke concentrations tended to increase before infections rose, suggesting that the first was driving the latter. The study authors also noted that they controlled for factors including overall virus prevalence, temperature, and a number of tests that were excluded by other studies that showed an association between wildfire smoke and COVID-19 infections in San Francisco and Orange County, California.

“Thus,” the study authors wrote, “we believe that our study greatly strengthens the evidence that wildfire smoke can enhance the spread of SARS-CoV-2.”

Converging Crises 

The 2020 wildfire season was not a typical fire season in the northern hemisphere. It was a record-breaking one. And the 2021 fire season already has the potential to be even worse, with more fires raging and acres scorched to date than in any year since record-keeping began in 1983. 

The severity and frequency of wildfires in the U.S. West have been widely attributed to the climate crisis, making the relationship between wildfire smoke and COVID-19 infections another example of how climate change can make other public health problems even worse. While not a climate scientist himself, Kiser notes that his study “would be a good example of how climate change can impact our daily lives.”

As smoke from Western fires now spreads across the U.S., does that mean we can expect to see another summer in which climate change exacerbates a global pandemic?

Kiser says such a conclusion would be “reasonable” if the relationship his team found between smoke and infections was indeed casual. However, there is one important difference between this year and last year: the existence of vaccines against the new virus. 

“Wildfire smoke is yet another factor,” Kiser says, along with the spread of the delta variant, “increasing the urgency of being vaccinated.”

In addition, he encourages people to take steps to protect themselves from inhaling smoke, such as avoiding outdoor exercise when PM2.5 concentrations are high.

“The takeaway from our study is that it's a good idea ... to reduce your exposure to wildfire smoke and COVID,” he concludes.