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 […]