If you find yourself in some secluded spot in the American West—maybe Yellowstone, or the deserts of Utah, or the forests of Oregon—take a deep breath and get some fresh air along with some microplastic. According to new modeling, 1,100 tons of it is currently floating above the western US. The stuff is falling out of the sky, tainting the most remote corners of North America—and the world. As I’ve said before, plastic rain is the new acid rain.

But where is it all coming from? You’d think it’d be arising from nearby cities—western metropolises like Denver and Salt Lake City. But new modeling published yesterday in the Proceedings of the National Academy of Sciences shows that 84 percent of airborne microplastics in the American West actually comes from the roads outside of major cities. Another 11 percent could be blowing all the way in from the ocean. (The researchers who built the model reckon that microplastic particles stay airborne for nearly a week, and that’s more than enough time for them to cross continents and oceans.)

Microplastics—particles smaller than 5 millimeters—come from a number of sources. Plastic bags and bottles released into the environment break down into smaller and smaller bits. Your washing machine is another major source: When you launder synthetic clothing, tiny microfibers slough off and get flushed to a wastewater treatment plant. That facility filters out some of the microfibers, trapping them in “sludge,” the treated human waste that’s then applied to agricultural fields as fertilizer. That loads the soil with microplastic. A wastewater plant will then flush the remaining microfibers out to sea in the treated water. This has been happening for decades, and because plastics disintegrate but don’t ever really disappear, the amount in the ocean has been skyrocketing.

Courtesy of Janice Brahney

In fact, this new research shows there may now be more microplastic blowing out of the ocean at any given time than there is going into it. Put another way: So much has accumulated in the ocean that the land may now be a net importer of microplastic from the sea. “That’s really highlighting the role of legacy pollution,” says Janice Brahney, an environmental scientist at Utah State University and co-lead author of the new PNAS paper. “The amount of plastics that are in our ocean is just overwhelming compared to anything that we produce in any given year in the terrestrial environment.”

These microplastics aren’t just washing ashore and accumulating on beaches. When waves crash and winds scour the ocean, they launch seawater droplets into the air. These obviously contain salt, but also organic matter and microplastics. “Then the water evaporates, and you’re left just with the aerosols,” or tiny floating bits of particulate matter, says Cornell University researcher Natalie Mahowald, who co-led the work with Brahney. “Classically, we atmospheric scientists have always known that there are sea salts coming in this way,” she continues. But last year, another group of researchers demonstrated this phenomenon with microplastics, showing that they turn up in sea breezes.

This time, Mahowald and Brahney thought bigger, using atmospheric models to show how far marine microplastics might travel after they take to the air. They also looked at other sources of microplastic emissions, like roads, cities, and agricultural fields. They knew, for instance, how much dust is generated from fields and how much microplastic might be in that dust.