Even in some of the most remote places on Earth, a fine rain of human-made debris pollutes the land and oceans. These microplastics are virtually everywhere—but the impacts to our health are still murky.

In an article published today in the journal Science, researchers stressed that building an understanding of how these particles are affecting us—especially the most tiny pieces—is crucial. We don’t know much yet about the threats posed by this pollution, but early findings reveal reasons to be concerned.

Microplastics are what they sound like: small pieces of what were once larger plastics. Plastics are forever; they stick around in the environment without degrading for potentially thousands of years. After time and exposure to sunlight, they tend to weather into smaller and smaller chunks with the same chemical composition. The main sources include car tires, clothes, and paint. Microplastics encompass numerous chemical compounds, come in all shapes, and range in size from visible bits of lint to the nano-scale.

microplastic collection
Microplastics collected by Dick Vethaak. © 2021 Dick Vethaak. All rights reserved.

Scientists are finding microplastics everywhere. Last year, a study found an average of 132 small pieces of plastic literally rained down onto each square meter of public lands in the western United States every year. They are even found in the snow and waters of the Arctic. In the ocean, organisms accumulate these pieces in their bodies over their lifetimes, sometimes to harmful or lethal extents, and in turn microplastic also shows up in the seafood we eat.

As unnerving as plastic-laden tuna may sound, researchers say that the main way humans are exposed to microplastics is probably through breathing. In the dust that swirls and settles around us, plastics are a significant presence. Think about your closet; most of your clothes that aren’t fully wool or cotton contain plastic. Everything from sweat-wicking Lycra to the polyester fibers of a cozy fleece is plastic, and those materials slough off over time. Tires also shave off little particles with every push of the brake pedal. And there’s various other sources, all winding up as airborne dust. One study found that the sky deposited between 575 to 1008 particles per square meter (that’s about 53.4 to 93.7 particles per square foot) across London every day.

While it’s clear that nowhere is safe from this cloud of microplastics, research into the human health impacts of the particles has been relatively scant. While laboratory experiments in animals have shown toxic effects at high concentrations, it’s hard to translate that to people. One review concluded that the available evidence doesn’t suggest widespread harm to humans—but added that the evidence is limited.

One problem is that we don’t know how much plastic humans are breathing in and eating. Especially for very small nano-size particles, it’s hard to measure microplastics in food, water, and in the body.

“It’s not easy to detect plastics [in humans] in comparison to, for example, metal or chemicals,” says Sinja Rist, a biologist studying microplastics at the Technical University of Denmark. That’s because plastics are mainly made up of carbon and hydrogen, very widespread elements. And the devices that can identify the plastics are limited in their resolution—they still can’t see most of the finer pieces that make their way into our bodies.

That’s important because the smallest particles are actually the most worrisome. Microplastics less than 10 microns in diameter have been shown in animal experiments to be able to cross into the blood and lymphatic system and accumulate in organs such as the kidney, liver, and brain.

These invisible fragments may be toxic on their own, and they also have the potential to harbor a layer of harmful microbes or chemicals. In epidemiological studies, researchers have found that working in factories with plastic-laden air is associated with increased risk of lung injury in workers. “We know that a very small fraction of these particles can enter your circulation,” says Dick Vethaak, an ecotoxicologist at VU University Amsterdam who co-authored the article. “But we don’t know how much, and what happens there.”

To understand the risks, scientists will need to quantify how much is entering our bodies, and whether that plastic is piling up in organs versus mostly being coughed or pooped out. Understanding how much plastic is sticking around as well as details about the pieces themselves—including their chemical composition and size—will help provide clues about their health impact. “Maybe there’s not much happening there … but maybe [the microplastics] have a unique pathology or toxicology,” says Vethaak. “If you don’t look for it, you don’t find it.” He says not looking into these potential effects “could really be a mistake.”

By the year 2025, scientists estimate 11 billion metric tons of plastic waste will have piled up in the environment. “It really boils down to our general consumption. It is plastic products that we use all the time that generate microplastics.” says Rist. Even if relatively benign, microplastics are just one part of the larger problem: how to handle mountains of garbage that last forever.