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A few years ago, Karen Clay and two other scientists studied the death rates for the 1918 flu pandemic in the U.S. 

Clay, a professor of economics and public policy at Carnegie Mellon University, was interested in whether more people died in cities with high levels of air pollution than those in cities with cleaner air.

They gathered mortality data from more than 180 cities in the U.S. — basically every city in the country, she said. They compared death rates between cities that used a lot of coal for electricity generation and those that used less. 

“Places with higher coal-fired capacity had more severe pandemics,” Clay said.

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Death rates were about 10 percent higher in cities with the dirtiest air, accounting for an additional 30,000 to 42,000 deaths in the U.S. Other studies have found similar effects for other outbreaks, like SARS in 2003. One study found patients with SARS in polluted regions were twice as likely to die from the disease as those in regions with cleaner air. 

For Clay, the results are further evidence air pollution plays a role in the spread and severity of infectious disease, especially respiratory infections like COVID-19. 

“The basic idea is that air pollution tends to irritate and inflame the respiratory system,” Clay said. “When you’re exposed to a virus, it makes it more likely that the virus can gain a foothold in your respiratory system. And so it makes it harder for your body to fight off that virus.”

Clay said that means people exposed to pollution are more likely “to have a more severe case and/or die” compared to those who’ve been breathing cleaner air. 

So far, Pittsburgh’s Allegheny County has had fewer cases than several counties near Philadelphia, and as of Friday at noon, only 2 of the deaths 90 coronavirus deaths in the state.

But the Pittsburgh region’s air fails to meet federal air quality standards, largely because of big industrial polluters like steel and coal plants. Clay worries that could mean more people get COVID-19, and those who do will suffer worse outcomes.

“I think that we would have a less severe outbreak if we had lower levels of air  pollution,” Clay said.

There’s plenty of biological evidence to support Clay’s fears, said Sally Wenzel, chair of environmental and occupational health at the University of Pittsburgh Graduate School of Public Health.

Wenzel said pollution can damage cells that line breathing passageways, which form the lung’s natural defense from foreign agents. 

“But when they’re damaged, they don’t function nearly as well as a barrier. And so things like viruses can get through that barrier and into the body, into the deeper spaces of the body,” Wenzel said.

That kicks off an inflammatory response that can lead to further infection. 

Wenzel said there are lots of other factors that weigh on the severity of the pandemic. Social distancing and hand-washing can slow the spread of the disease; and those who smoke or have underlying health issues like diabetes could be at greater risk of illness or death. 

But scientists are also studying whether the air in the industrial city of Wuhan, China and in Milan, Italy, which has some of the most polluted air in Europe, made the pandemic in those cities worse. 

The outbreak started in Wuhan, a city of 11 million people that the Chinese government eventually shut down to try to control the disease’s spread; Milan is the capital of Lombardy, the hard-hit Northern Italian region that accounts for more than half of that country’s COVID-related deaths. 

The air in both cities has cleared up as governments have locked down economies to slow the spread of the virus. That’s also happening in the U.S. 

Pollution in Pittsburgh, for instance, has “flatlined” since stay-at-home orders have shut down large parts of the economy, said Albert Presto, an associate research professor in mechanical engineering at Carnegie Mellon. 

Presto, who’s been watching air quality monitors around Allegheny County, said those drops are especially detectable during morning and evening rush hours. 

“Typically, you know, you see a big bump (in pollution) every morning from people going to work,” he said. “That’s pretty much disappeared the last couple of weeks.”

In addition to traffic going down, coal-fired electric generation has also plummeted during the pandemic. PJM Interconnection, the regional grid operator in the mid-Atlantic, said coal-fired power generation declined by nearly 50 percent from March 2019 to March 2020. 

PJM spokesman Jeffrey Shields said part of the decline may be traced to warm temperatures this March, which were among the warmest on record in parts of the U.S. 

Jim Kelly, deputy director of environmental health at the Allegheny County Health Department, said U.S. Steel’s Clairton Plant has slowed its production of coke, a key component of steelmaking. The plant — Allegheny County’s largest single source of particle and sulfur pollution — is lengthening the amount of time it processes coke, which limits emissions from the process, Kelly said. He said that decision was in response to market demands. 

Scientists say they don’t know yet whether a few weeks of suddenly cleaner air will blunt the impact of the virus. Those answers will only come when the pandemic ends. 

This story is produced in partnership with StateImpact Pennsylvania, a collaboration among The Allegheny Front, WPSU, WITF and WHYY to cover the commonwealth's energy economy.