Air Pollution Plummeted During the Pandemic, Except in Areas With Wildfires

Los Angeles was on its way to join that list with one of its cleanest years in decades through the first half of 2020. Then wildfires burned a record 4.3 million acres in California beginning last summer, including three that raged at the same time around America’s second most populous city. L.A. ended up with a 15% increase in particulate air pollution, according to IQAir.

Similarly, air pollution rose 35% in San Francisco, 38% in Portland, Ore., 53% in Phoenix and 18% in Salt Lake City.

Dirty West

Smoke from wildfires caused air pollution to rise in major cities in the Western U.S., while it fell in most of the East

Fine inhabitable particles (PM2.5), commonly produced by wildfires and dangerous for humans in large densities

WHO guideline level

for safe Air quality

Unhealty for

sensitive groups

Fine inhabitable particles (PM2.5), commonly produced by wildfires and dangerous for humans in large densities

WHO guideline level

for safe Air quality

Unhealty for

sensitive groups

Fine inhabitable particles (PM2.5), commonly produced by wildfires and dangerous for humans in large densities

WHO guideline level

for safe Air quality

Unhealty for

sensitive groups

Fine inhabitable particles (PM2.5), commonly produced by wildfires and dangerous for humans in large densities

WHO guideline level

for safe Air quality

Unhealty for

sensitive groups

Smoke from wildfires also drove pollution increases in Melbourne, Australia, where the readings soared 27% and in Buenos Aires, where it rose 15%.

“This really shows the alarming scale of the wildfires,” said Meghan Thurlow, vice president of sensing systems and applied science for Aclima, a San Francisco firm that measures air pollution.

Cities on the other side of the U.S. mirrored the global declines. Air pollution fell 13% last year in Chicago, 9.5% in Philadelphia and 7% in New York.

The worst air quality overall was still in Asia last year. India’s New Delhi and the surrounding region had the dirtiest air of all the world’s capital cities measured, according to IQAir, and 49 of the 50 most polluted cities measured world-wide were in Bangladesh, China, India and Pakistan.

Air pollution accounts for seven million premature deaths annually, the World Health Organization has estimated. With life returning to normal around the globe, air pollution levels linked to human activity will likely increase again, IQAir officials say.

Even before Covid-19, the world’s biggest economies were already trying to reduce air pollution amid pressure to reduce carbon emissions.

In China, where the Covid-19 pandemic began, 86% of the country’s cities recorded cleaner air in 2020 than in the previous year. Half that improvement was attributable to the lockdowns early last year, while the other half was due to China’s long-term pollution fighting measures, said Lauri Myllyvirta, lead analyst at the Center for Research on Energy and Clean Air.

China’s air quality has been steadily improving since 2013, when the government began, among other measures, shifting from coal to gas heating.

The lowering of emissions world-wide was “an unplanned scientific experiment” that showed how quickly air pollution levels can be reduced, said Glory Dolphin Hammes, chief executive of IQAir North America, an IQAir subsidiary. “What we have been able to determine is we can do it,” Ms. Hammes said.

But when fires start in California, Australia and elsewhere, they grow faster than in the past and create an unprecedented public health threat, according to researchers. The fine particulates they generate are of the most concern, because they often contain incinerated pipe, plastic and other household materials, said Lisa Miller, a professor at the University of California Davis School of Veterinary Medicine.

“We are thinking perhaps the particulate matter from these fires may be more dangerous than particles from an automobile,” Dr. Miller said.

Write to Jim Carlton at jim.carlton@wsj.com and Sha Hua at sha.hua@wsj.com