Two main risk factors are currently known to raise the chances of dying from the coronavirus that has brought the world to a halt: being old and having a weak immune system. Air pollution makes the second of those more likely.

Researchers at Harvard University have found that coronavirus patients in the most polluted areas of the United States are more likely to die from the illness than those in cleaner areas, based on an analysis of 3,080 counties – which represent 98% of the population.

The COVID-19 could kill between 100,000 and 240,000 Americans, according to official government estimations. More than 30 states have taken lockdown measures to prevent the spread of the virus, while others have been reluctant to do as well, so far. There are over 400.000 confirmed cases as of writing this in the US.

The new study showed a correlation between long-term exposure to air pollution and heightened death rates associated with the virus. The findings could impact how medical resources necessary to respond to the virus are being distributed throughout the country.

“There is a large overlap between causes of deaths of COVID-19 patients and the diseases that are affected by long-term exposure to fine particulate matter (PM2.5),” the study reads. “PM2.5 contains microscopic solids or liquid droplets that are so small they can be inhaled and cause serious health problems.”

The new analysis demonstrated that even slight increases in the level of particle pollution had negative impacts associated with COVID-19. Someone who has lived for decades in a county with such dangerous levels of pollution is 15% more likely to die from the coronavirus than an individual in a cleaner area.

The researchers collected data on particulate matter from more than 3,000 counties over the past 17 years. They compiled COVID-19 death statistics through April 4 from each county, using data from Johns Hopkins University’s Center for Systems Science and Engineering.

“The results of this study also underscore the importance of continuing to enforce existing air pollution regulations during the COVID-19 crisis. Based on our result, we anticipate a failure to do so can potentially increase the COVID-19 death toll and hospitalizations, further burdening our healthcare system,” the study reads.

Air pollution is the most urgent environmental health risk in the world. More than 90% of the planet breathes unhealthy air, leading to seven million premature deaths per year and billions of dollars in costs for health services.

The world is facing a “pandemic of air pollution”, which shortens life expectancy by almost three years — more than tobacco, AIDS, wars, or diseases such as malaria, according to a recent study. East Asia and Africa are the most affected regions.

Research on previous outbreaks has also suggested bad air makes viruses more deadly and spread further. A study of SARS-CoV-1 victims in 2003 found that patients were twice as likely to die in regions where air pollution was high rather than low.