It feels odd to say anything bad about composting — our favorite soil-enriching way to dispose of food waste — but sometimes the news works that way.

According to a study published in Science Advances, composting can be an easy pathway for microplastics, particles of plastic less than 5 millimeters in size, to enter the environment.

By land and by sea

While we know these tiny particles are found in the oceans, they are creeping into our land and air as well — we just aren't paying nearly as much as attention to them.

That's an odd truth, points out University of Toronto ecology and evolutionary biologist Chelsea Rochman, since plastic originates on land, after all.

"More recently, however, researchers have expanded their focus to include freshwater and terrestrial environments. This is a welcome development," she writes in a commentary about microplastics for Science, "given that an estimated 80 percent of microplastic pollution in the ocean comes from land and that rivers are one of the dominant pathways for microplastics to reach the oceans."

Such studies expand our understanding of where microplastics are cropping up in our environment. The closer to the source we get, Rochman argues, the better we'll be able to manage microplastics as a scourge. This is especially important since the effects on microplastic particles (MPPs) on our bodies isn't fully understood.

"Microplastic research must be global and include a greater understanding of the scale, fate, and effects of microplastic pollution at all stages, from its sources via freshwater and terrestrial ecosystems to its ocean sink," she concludes.

Fertilizing with plastics

A truck spreads fertilizers on a field in Germany with wind turbines stand on the horizon Germany has strict regulations about fertilizer quality, but microplastic particles can get still get into the system. (Photo: RikoBest/Shutterstock)

A study published in Science Advances addresses one particular corner of this understudied problem: composting. Specifically, the researchers looked at the household and food industry biowaste collected by different composting plants in Germany. These plants use biowaste to create biogas for electricity and to create fertilizers for agriculture. (Food waste composting on a large scale to make fertilizer is much more popular in Europe than it is in the U.S., but it's catching on.)

What the researchers found is much of the collected biowaste has some sort of plastic contamination. Households, for instance, didn't do a good enough job sorting their plastics from their compost materials, or they introduced plastics unnecessarily to the process.

"What happens most of the time is that people don't like to put garbage into the bin as it is. They like to wrap it up," Ruth Freitag, a chemist at the University of Bayreuth in Germany, and a coauthor of the study, tells NPR.

The food industry was generally better about this than households, but still had its own set of problems. Unsold food items would enter the biowaste plants wrapped in plastics or with their sale stickers still on them. Most, however, had "secondary" microplastic particles, the result of packaging materials breaking down.

A wooden compost bin with a sign on it reading 'Compost bin in use' Composting is a great way to use food waste, but only if its free of other materials. (Photo: mubus7/Shutterstock)

The biowaste goes through a filtering and sieving process once inside the plants in an effort to reduce microparticles. Additionally, the composting process can reduce the presence of particles, depending on a variety of factors, including the weather and the type of composting process the plant uses. Nonetheless, particles were still found in the fertilizers the researchers tested.

"We recorded particle counts varying from 14 to 895 particles per kilogram dry weight," the researchers wrote.

These microplastic particles "inevitably" end up in the environment. Whether it's in the food we eat, or in the worms that consume the soil. Agricultural runoff will also carry the particles into different parts of the environment, including, of course, the ocean.

It's just one more potential source for us to be aware of as we try and reduce the presence of microplastics in all aspects of our environment.