As the SARS virus tore through China in 2003, Zuo-Feng Zhang wondered whether the countrys notoriously polluted air might be amplifying its dangers. The answer he and his scientific colleagues found feels frighteningly relevant now, not just for China, but all across a world where SARS coronavirus cousin is bearing down on billions of people who already live with unhealthy air.

Sifting through data from five different regions, Zhangs team concluded that SARS patients living in the most polluted places were twice as likely to die from the disease as those in the cleanest areas.

We found a very strong correlation between air pollution and deaths from the virus, said Zhang, an epidemiologist at UCLAs Fielding School of Public Health. The effect held not just for pollution levels at the time of the outbreak, but for levels over the previous two years as well, indicating prior exposure had likely compromised peoples ability to fight off the illness.

It was a small study, Zhang cautioned in a phone interview, since SARS took far fewer lives than the current pandemic. But the findings, which, given the diseases similarities, almost certainly apply to COVID-19 too, are consistent with everything scientists already know about air pollutions mostly invisible toll on health.

It stifles the immune response to infections, it actually directly can damage immune cells, said Aaron Bernstein, interim director of the Center for Climate, Health, and the Global Environment at the Harvard T.H. Chan School of Public Health. It also tends to be inflammatory in the lung, and that inflammation can interfere with mechanisms that clear pathogens (including viruses) from our respiratory tracts.