Record-breaking wildfires made North American air worse in 2020

forest fire
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The devastating wildfires that torched a record 4.3 million acres in the U.S. in 2020 made North America the only region in the world where air quality was worse than during the previous year.

Most of the world's major cities had better air last year than in 2019, mainly thanks to coronavirus lockdowns that cleared skies for weeks or months, according to the annual report by air quality platform IQAir. Though India's New Delhi was the most-polluted capital , its levels nevertheless fell 16% from the previous year.

Los Angeles, Melbourne and Sao Paulo, all of which were close to major wildfires in 2020, were among the few where pollution was worse than in 2019.

In the U.S., 38% of cities had levels of fine particulate matter, known as PM2.5, that exceeded World Health Organization quality standards in 2020, up from 21% the previous year. Skies in the western coast were so choked with wildfire smoke during September that 77 of the world's 100 most polluted cities that month were in the U.S.

High exposure to PM2.5, which is emitted by fossil-fuel powered vehicles, and biomass burning, can cause asthma, lung cancer, heart disease, and has been associated with low birth weight, acute respiratory infections and strokes, IQAir said.

An increasing amount of studies are also linking worsening to increased vulnerability to COVID-19, which attacks respiratory and cardiovascular systems. Coronavirus deaths attributable to long-term air pollution exposure could represent between 7% and 33% of all fatalities due to the virus, according to an early study published in Cardiovascular Research and cited by IQAir.

"We know that when there is a wildfire, there will be increases in respiratory and cardiac events. We see increases in ER visits and hospitalizations for things such as asthma, exacerbation, for pneumonia, for acute bronchitis," said Dr. Mary Prunicki, who studies the impacts of air pollution on allergies and the immune system at Stanford University. The 2020 fire season "was the worst air quality the (San Francisco) Bay Area has ever experienced, and for the longest duration."

The San Francisco Bay Area saw a total of 25 days with unsafe levels of particulate matter pollution in the air for a 24-hour period, a record according to the Bay Area Air Quality Management District. Residents felt the impact of this air pollution in a visceral way on Sept. 9, when wildfire smoke combined with clouds and fog to block out the sun and cast an eerie, "apocalyptic" orange hue over the sky.

The same fumes also contain greenhouse gases that are warming the planet and in turn are making wildfires more frequent and intense. Elsewhere, once-in-a-decade sandstorms in China's northwestern Xinjiang province polluted cities in the region for months and sent dust as far as Beijing. The desert oasis city of Hotan had the highest particle pollution worldwide for several months because of the sandstorms, according to IQAir.

Overall, 86% of Chinese cities had cleaner air last year, but are returning to prepandemic levels, according to satellite data published Monday by the European Space Agency.

IQAir's analysis also suggests that as cities tightened lockdowns last year, pollution tended to improve, and these gains were lost when movement restrictions eased. This indicates 2020's improvements will prove fleeting without significant changes to the energy mix and to human behavior.

Wildfires and other discrete events further complicate the idea that a mass reduction in certain human activity is all that's needed to improve , Dr. K. Max Zhang, a professor at Cornell University who researched pollution in six Chinese cities during lockdown. While he finds hope in the fact that most countries did see a reduction in particulate pollution last year, he says a larger climate mitigation is needed.

"COVID in some ways gave us a reality check," Zhang says. "If our efforts are concentrated just on cutting down travel, for example, that probably won't get as much benefit as we had hoped."


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Journal information: Cardiovascular Research

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