In oceans and rivers, in soils and sand, in the bodies of animals and humans lurk tiny pieces of plastic, often too small to be seen by the naked eye. Now, as NPR’s Christopher Joyce reports, a new study has found microplastics in a remote and largely untouched region of the Pyrenees mountains—a sobering revelation of just how far the ubiquitous pollutants can travel through the atmosphere.

Microplastics are less than five millimeters long and derive from a variety of sources, including larger plastics that do not quickly biodegrade, but instead break down into smaller and smaller pieces. These little fragments of plastics have previously been found in the air over large cities like Paris and Dongguan in China. But scientists were surprised to discover that microplastics had infiltrated far beyond urban areas to the pristine mountains that border France and Spain.

Writing in the journal Nature Geoscience, a team of researchers from Scotland and France explains that they analyzed samples of dust, rain and snow collected at the Bernadouze meteorological station over the course of five months. The station sits 4,500 feet up the mountains in southwestern France and is 75 miles away from Toulouse, the nearest city. And yet, the site was inundated with microplastics—an average of 365 plastic particles fell each day on a square meter collector.

“It’s astounding and worrying,” says Steve Allen, a PhD student at the University of Strathclyde in Glasgow who led the new research with Deonie Allen of EcoLab in Toulouse.

The samples included different types of plastics: clothing fibers, fragments of plastic bags and pieces of packaging material. Using computer simulations, the team determined that the particles floated from at least 100 kilometers (about 62 miles) away, reports Science’s Alex Fox. But it’s entirely possible that they came from more distant locations. The area around the collection site is sparsely populated, with no major industrial or commercial centers. What’s more, scientists discovered a “visible quantity of orange quartz-like fine dust” that they believe blew in from the Sahara. “The fine dust and other particulate matter that potentially include some [microplastic] particles are possibly Saharan-, North African- or Iberian-sourced material,” the study authors write.

The new research suggests that it is not only city dwellers who are at risk of breathing in large quantities of microplastics. Just what this means in terms of impacts on human health is unclear. Microplastics seem to negatively affect the animals that are exposed to them; studies have shown that the particles impair reproduction and damage the digestive tracts of various species. But more research is needed to determine potential risks for humans.

There is, in fact, much about microplastic pollution that is poorly understood. “The drivers in plastic degradation are fairly well known,” says study author Deonie Allen, “but the transport drivers and mechanisms–especially atmospheric transport–for microplastic appears to be complex and an area of research that now needs to be unravelled.”

How to tackle the pervasive microplastic problem is another difficult question. Once these tiny particles make it into the environment, it’s really hard to get rid of them. Producing less plastic, and keeping larger plastic objects out of the environment in the first place, might be the best way to curb the ever-expanding source of pollution.