Global warming doesn’t just mean rising seas, soaring temperatures, and prolonged droughts. It means a miasma of polluted air, hovering over a city near you. Air pollution and climate change have always been linked—the greenhouse gases frying the globe are emitted from power plants and cars alongside more traditionally toxic pollutants—but a new study confirms some surprising new evidence that a hotter planet will almost certainly be a smoggier one.

Climate change is beginning to seriously impact atmospheric circulation and the hydrological cycle. Largely, that means increasingly lighter winds and less rain; and that lot of the pollution that would ordinarily be dispersed will instead hang stagnant. And a decent puff of it will end up in our lungs.

That’s a crude summary of the finding of an important new study just published in Nature Climate Change, which determined that 55 percent of the globe’s population will be effected by increased air stagnation wrought by climate change. The report singles out India, Mexico, and the western US as locales where the polluted stagnation will be “particularly acute… owing to the intersection of large populations and increases in the persistence of stagnation events, including those of extreme duration.”

To be clear, the air stagnation imminent in our climate-changed future isn’t dangerous on its own. But the vast majority of urban areas are great hives of air pollution. And the billions of people who live in these dense, industrialized areas depend on regular rain and strong enough winds to wash it all away. Otherwise, the airborne particulates and toxins will just float there, invading lungs and Beijing-ifying the streets.

Consider: the reason that cities in California’s Central Valley regularly top the ‘nation’s most polluted’ lists isn’t that they’re exponentially dirtier than other polluting hubs. It’s the unique geography; it’s trapped on three sides by mountains, which creates a ‘pool’ for the air to stagnate in. Rain and wind are sparse, to boot. Sure, there’s lots of industrial and agricultural pollution, too—but the air is worse because it stagnates.

Now imagine a similar smog stagnation gripping, no mountains needed, some 55 percent of the world’s population centers. It’s a disturbing thought, considering air pollution already causes around 7 million premature deaths a year, according to the World Health Organization.

And that’s one reason this study is so important to the climate change conversation—it underlines the public health threat posed by climbing temps. When Obama was touting the EPA’s new carbon regulations, he emphasized the public health benefits of drawing down emissions: It would reduce asthma and respiratory illness, he pointed out. But that’s largely because shuttering dirty power plants cuts both carbon and particulate pollutants simultaneously; fighting climate change also means fighting asthma.

Now, scientists have demonstrated there’s an additional layer of concern to grapple with on the pollution front; climate change is going to begin blocking cities’ toxic release valves. If we don’t work to slow carbon emissions, these steamier cities will find their streets clogged with stagnant smog. Scrubbing that pollution and finding novel ways to clear the air, too, then, will prove to be a pressing concern in the not-so-distant future.