As the novel coronavirus tears around the world, it’s exploiting our biggest weaknesses, from creaking health care systems to extreme social inequality. Its relationship with one pervasive and neglected problem, however, is more tangled: Air pollution has intensified the pandemic, but the pandemic has—temporarily—cleaned the skies.

When new evidence emerged this week that dirty air makes COVID-19 more lethal, it surprised no one who has followed the science of air pollution—but the scale of the effect was striking. The study, which must still undergo peer review for publication, found that the tiny pollutant particles known as PM2.5, breathed over many years, sharply raise the chances of dying from the virus.

Researchers from Harvard University’s T.H. Chan School of Public Health analyzed