As bigger pieces of plastic get broken down into smaller and smaller pieces, the pollution becomes harder and harder to see -- but that doesn't mean it's not there.

In a new study, published Monday in the journal PNAS, scientists looked at the different ways tiny plastic particles, or microplastics, can become airborne and transported via the atmosphere at global scales, infiltrating a diversity of ecosystems along the way.

Much of the research into the transport of plastic pollution has focused on ocean currents, but there are other modes of travel.

As previous studies have made clear, airborne microplastics are being deposited in some of the remote places on Earth, from Arctic glaciers to the peaks of the Pyrenees.

According to the latest study, most of the plastic particles that make their way into the atmosphere are sourced from decades-old, broken-down waste -- food wrappers, soda bottles and plastic bags.

"Microplastic particles and fibers generated from the breakdown of mismanaged waste are now so prevalent that they cycle through the earth in a manner akin to global biogeochemical cycles," researchers wrote in their paper.

For the study, researchers at Utah State University used both the predictions of an atmospheric transport model and their own observations of microplastic deposition to better understand the sources of airborne plastic pollution.

The efforts showed that roadways are the primary source of airborne microplastics.

Everyday, the tires of large trucks catapult tiny bits of plastic in the sky. These tiny plastic particles can be quickly carried aloft by winds, joining what scientists call "the atmospheric limb of the plastic cycle."

In the ocean, waves do the catapulting. The constant churn of the sea regularly lofts aerosolized particles in the sky, before ocean breezes carry the bits of plastic in the atmosphere.

According to the new study, microplastics are also regularly introduced to soil via fertilizer treatments. These bits of plastic can become airborne during the tilling and harvesting processes, as the soil is churned by heavy machinery.

Once airborne, scientists estimate microplastics can remain in the atmosphere for nearly seven days, enough time to cross an ocean or a continent.

Scientists identified hotspots for land deposition of airborne microplastics in the United States, Europe, Middle East, India and Eastern Asia.

The analysis showed airborne plastics also regularly end up in the Pacific Ocean and Mediterranean, as well as the coastal waters of southern Australia.

Plastic dust from agricultural sources more frequently impacts human populations and ecosystems in Africa and Eurasia, while roadway-generated microplastics impact populated regions all over the world.

"At the current rate of increase of plastic production -- approximately 4 percent per year -- understanding the sources and consequences of microplastics in the atmosphere should be a priority," researchers wrote.

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