More than 1,000 tons of microplastic rains in protected areas across the western United States every year the equivalent to more than 123 million plastic bottles new research suggests. Long-term accumulation of small pieces of plastic measuring less than 5 millimeters long is reminiscent of the global dust cycle but distinctly human in origin, posing potential consequences for vulnerable ecosystems around the world.

Microplastics are small pieces of plastic that began its lifecycle as a larger piece that has fragmented over time. Previous research has shown that its raining plastic in Americas Rocky Mountains and snowing microplastics in the Arctic and with more than 340 metric tons of plastic produced in 2017, the world shows no signs of global production slowing down. As such, it is estimated that 11 billion metric tons of plastic will have accumulated in the environment within the next five years.

Using high-resolution atmospheric deposition data, researchers at Utah State University set out to identify samples of microplastics and other particulates collected over 14 months in 11 national parks and wilderness areas located in Arizona, Wyoming, Idaho, Colorado, California, Utah, and Nevada. Pieces were compared by their size and shape and identified through their composition in order to identify sources of plastic emitted in the atmosphere, whether they moved through dry or wet conditions, and where they were ultimately deposited.

"We were shocked at the estimated deposition rates and kept trying to figure out where our calculations went wrong," said study author and Assistant Professor Janice Brahney in a statement. "We then confirmed through 32 different particle scans that roughly 4 percent of the atmospheric particles analyzed from these remote locations were synthetic polymers.

Plastic was present in 98 percent of all wet and dry samples amounting to a daily deposition rate of 132 plastics per square meter. Most plastic deposited through precipitation was sourced from cities and areas with high population densities while, on the other hand, plastics that were deposited under dry conditions showed indicators of having traveled long distances through atmospheric patterns, in some cases across continents.

"Several studies have attempted to quantify the global plastic cycle but were unaware of the atmospheric limb," said Brahney. "Our data show the plastic cycle is reminiscent of the global water cycle, having atmospheric, oceanic, and terrestrial lifetimes."

Most fibers deposited in both dry and wet conditions were those from clothing and industrial materials; approximately 30 percent were brightly colored acrylic microbeads likely derived from industrial paints and coatings rather than personal care products, while 4 percent of atmospheric particulates identified from remote locations were plastic polymers. Even so, the study authors add that their findings likely underestimate the actual pervasiveness of environmental microplastics as they didnt count clear or white particles.

"This result, combined with the size distribution of identified plastics, and the relationship to global-scale climate patterns, suggest that plastic emission sources have extended well beyond our population centers and, through their longevity, spiral through the Earth system," write the study authors in Science.

Plastic has become ubiquitous in the environment yet consequences for human and environmental health remain largely unknown. Understanding how the pollutant is transported through the environment is an important mechanism in understanding how microplastics circumnavigate the globe, particularly in vulnerable environments.

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Over 1000 Tons Of Plastic Raining Down On US National Parks And Protected Areas Each Year - IFLScience