Polluted air caused by smoke released from the record-breaking wildfires in the US last year has been linked to a strong increase in covid-19 cases and deaths.
Francesca Dominici at Harvard University and her colleagues say 19,742 recorded covid-19 cases and 748 covid-related deaths can be linked to spikes in tiny particulate matter, PM2.5, released by the blazes in California, Oregon and Washington.
Links between long-term exposure to dirty air and greater risk of death and severe illness from covid-19 have already been well-documented. But the new research puts numbers on how short-term exposure to pollution, in this case from wildfires, may have made the pandemic’s health impact worse.
“What this is saying is, number one, especially for the counties affected by wildfires, people should absolutely get vaccinated and wear a mask,” says Dominici.
The team looked at daily data on covid-19 cases and deaths and PM2.5 levels between March and December 2020 in 92 counties which cover 95 per cent of the population in California, Oregon and Washington. They then accounted for other possible explanations for links, including looking at the weather and Facebook data on how much people moved around, and considered a counterfactual world without the fires.
Across the counties as a whole, they found each extra 10 micrograms of PM2.5 per cubic metre of air over 28 days was linked to an 11.7 per cent increase in coronavirus cases, and a 52.8 per cent increase in covid-19 deaths. Some counties saw PM2.5 levels higher than 500 micrograms per cubic metre for days in a row due to fires, well above the level deemed “hazardous” by US environmental authorities.
The impact of pollution on covid-19 cases and deaths varied widely between areas. Dominici says that is probably because “the trajectory of the pandemic within each county was very, very different”. The team thinks cases increased due to PM2.5 exposure because it led to more severe illness. This might also have had an impact even on people with mild illness. For instance, people with what would ordinarily have been an asymptomatic infection might have developed symptoms.
There are some caveats. Dominici says there may yet be other explanations for the link that the team didn’t account for. And the amount of PM2.5 people were estimated to be exposed to, using smoke satellite images, may not reflect their true exposure.
Nonetheless, the research implies there is another motivation to cut the carbon dioxide emissions which are projected to worsen wildfires in the western US as the world warms. “This also provides another reason why tackling climate change is so important,” says Dominici.
Journal reference: Science Advances, DOI: 10.1126/sciadv.abi8789
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