New York City's skyline on a smoggy day in May 2019. Photo: Drew Angerer/Getty Images

COVID-19 is underscoring the connection between air pollution and dire outcomes from respiratory diseases.

Why it matters: Old-fashioned air pollution is almost certainly the single biggest environmental health threat, contributing to the deaths of some 7 million people a year according to the WHO, making it comparable to deaths from smoking.

How deadly COVID-19 might be is a function of a number of variables, from the age of a patient to viral load to how overwhelmed a hospital system might be. But a growing body of research is a reminder of the hidden health consequences of living with serious air pollution.

Driving the news: A new study by researchers at Harvard T.H. Chan School of Public Health examined more than 3,000 counties in the U.S. and found that higher levels of the small particulate matter known as PM2.5 were associated with higher rates of death from COVID-19.

  • Some of the results were startling: A person who had lived for decades in areas with high levels of PM2.5 was 15% more likely to die from COVID-19.
  • On the whole, the paper showed a clear statistical relationship between air pollution and dire outcomes from COVID-19.

Other early studies have shown a similar relationship in countries hit hard by COVID-19.

  • One analysis published in Environmental Pollution found a connection between the high rates of death from the pandemic in northern Italy, where as many as 12% of patients have died from the disease, and the region's air pollution, which is some of the worst in Europe.

Flashback: In 2003, researchers found that SARS patients in the most polluted parts of China were twice as likely to die from the virus as those living in low pollution areas. And a study of the 1918 flu pandemic found American cities that burned more coal for electricity experienced more excess deaths than cities that didn't use as much coal.

How it works: None of the studies tried to answer why exposure to air pollution might worsen outcomes for COVID-19, but many doctors believe fine air particulate matter and ozone will damage the lungs just as smoking would, causing inflammation and leaving them less able to fight off infection of any sort — including from the novel coronavirus.

  • Hospitals frequently experience an increase in admissions for pneumonia a few days after air pollution alerts.
  • "It absolutely makes sense that we would see a synergistic effect between air pollution exposure and worse outcomes for COVID-19," says Jack Caravanos, a clinical professor of environmental health sciences at New York University.
  • Air pollution could also be one factor in another statistical anomaly emerging from the COVID-19 pandemic: unusually high death rates for African Americans, who are more likely to live in counties with persistent air pollution problems.

The big picture: While many countries, including the U.S., have generally experienced improvements in air quality over recent decades, scientists are learning that even lower levels of air pollution are still a threat.

Yes, but: The irony is the near-global shutdown triggered in response to the pandemic has helped lead to a marked improvement in air quality in polluted cities like Los Angeles and New Delhi. But that's only temporary — and a few weeks of cleaner air won't make up for decades of smog.

The bottom line: If COVID-19 has taught us anything, it's this: Our lungs matter. Perhaps when the economy restarts post-pandemic, we'll take greater care of the air around us.

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Coronavirus dashboard

Illustration: Annelise Capossela/Axios

  1. Global: Total confirmed cases as of 11:30 p.m. ET: 20,014,574 — Total deaths: 734,755 — Total recoveries — 12,222,744Map.
  2. U.S.: Total confirmed cases as of 11:30 p.m. ET: 5,089,416 — Total deaths: 163,425 — Total recoveries: 1,670,755 — Total tests: 62,513,174Map.
  3. Politics: Trump claims he would have not called for Obama to resign over 160,000 virus deathsHouse will not hold votes until Sept. 14 unless stimulus deal is reached.
  4. Business: Richer Americans are more comfortable eating out.
  5. Public health: 5 states set single-day coronavirus case records last week — A dual coronavirus and flu threat is set to deliver a winter from hell.
  6. Sports: The cost of kids losing gym class — College football is on the brink.
  7. World: Europe's CDC recommends new restrictions amid "true resurgence in cases."