People exposed to air pollution are more likely to die from COVID-19 than people living in areas with cleaner air, according to a new study. Though the study’s findings focus on the United States, they align with similar results from Italy.

This research is an ominous sign for many developing countries, where air pollution levels often far exceed World Health Organization guidelines. More worrying still, air quality inside people’s homes can be magnitudes worse than the air they breathe outside, due in large part to how people cook.

Globally, almost three billion people still rely on open fires or inefficient stoves to cook their food, filling their homes with dangerous levels of smoke. It is well documented that household air pollution from cooking increases susceptibility to respiratory infections such as pneumonia and aggravates respiratory illnesses like asthma – which may, in turn, lead to poorer outcomes after a COVID-19 infection.