Everyone loves the feeling of clean clothes—except maybe sea animals. Each load of laundry you do may be pouring hundreds of thousands of tiny pollutants into the water, which are then ingested by clams, mollusks and other sea creatures around the world.

Microfibers, or tiny bits of polyester and acrylic clothing less than 1/5 of an inch long, along with microplastics and microbeads—exfoliants found in beauty products—form when larger pieces of plastic break down. These tiny pollutants are among the most prevalent sources of marine pollution, and they may spell trouble for ocean and freshwater creatures.

But a new study shows that there’s hope: Some clothes, namely those made from acrylic, are much more polluting than others. Clothes made from synthetic materials are the main source of microfibers—and microfibers, while less well-known than microbeads, are far more prevalent. “Microfibers are actually the most prevalent type of microplastic that we’re seeing across the board,” says Katherine O’Reilly, a Ph.D. student at the University of Notre Dame.

And those clothes, it turns out, can be hugely variable when it comes to how many fibers they shed. “Some fabrics were releasing up to 3 times more fibers than others,” says Richard Thompson, a professor at Plymouth University in the U.K. and co-author of the new study, which was published online September 25 in the journal Marine Pollution Bulletin. “It does suggest that there are things manufacturers can do to reduce the numbers of fibers [released].”

For the new study, Thompson and Imogen Napper washed fabric samples of different types: acrylic, polyester and a polyester-cotton blend. Then, they filtered the washing machine’s wastewater to count the fibers. They found that acrylic cloth, found in clothes from sweaters to microfleece jackets, sheds fibers three to four times faster than the poly-cotton blend. For instance, if you washed 6 kilograms of the same fleece, 700,000 fibers per load of laundry could be dumped into the wastewater stream.

Some of those fibers are filtered out at a wastewater treatment plant, but others get through; one estimate is that 40 percent of fibers slip through a treatment plant to the open water. When they reach rivers, lakes or the ocean, they can be ingested by filter feeders like clams and mussels—and have devastating effects.

“The simple act of ingesting microplastics can make animals feel full without giving them nutrition,” says O’Reilly. “They eat, but they starve.” Or, says NOAA scientist Sherry Lippiat, toxins and bacteria tend to accumulate on the plastic, which can then be ingested by the animals. “We’re really concerned about the association between plastics and these chemical contaminants, but we’re not sure how much of a source plastics are of these contaminants.” Neither Lippiat nor O’Reilly were involved in the current study.

There are still many unknowns when it comes to microplastics. For instance, scientists don't kow whether these pollutants choke animals, though Lippiat calls it "a likely possibility." They also don't know how long microplastics stay in an animal or whether they will accumulate up the food chain—from clam to crab to a fish to human, for example. But it is known that they are ubiquitous, and they are not going away. “We’re finding it everywhere we’ve looked,” says Lippiat.

Indeed, microplastics have been found in rivers, lakes, at the bottom of the ocean and even in a remote lake in Mongolia.

Says Thompson: “We need to recognize that plastics are a persistent pollutant. Even if tomorrow we could wave a magic wand and stop all pollution of plastics to the environment, we'd still see for decades an increase in the plastic out there because of the fragmentation of the larger pieces that are out there ... While there's not cause for alarm right now, there might be if we continue with business as usual.”

The lesson, according to Thompson, is not that wastewater treatment plants should step up their game: that’s not the point. “You've got to consider, what do you do with the sewage you captured?” Sewage sludge—the leftover "solids" from a wastewater plant, now full of tiny plastic particles, too—is landfilled, incinerated or treated and used as fertilizer. That means that in most cases, the microplastics captured in a filter will just escape into the environment again.

Rather, he says, we need to address the problem at the source. “We're advocating that manufacturers take into account not just the appearance of the garment but also the longevity of the garment.” After all, a shirt that sheds fibers three times as quickly will wear out three times as quickly.

Clothing manufacturer Patagonia, which funded a study into clothes and microplastics a few years ago, recently announced on its blog that it was taking a number of steps to minimize the problem. The company said it was asking washing machine manufacturers to research how they could reduce shedding or trap fibers and "explor[ing] ways to integrate criteria to assess shedding of synthetic microfibers into … new materials within our product line." It also asked customers not to buy "what you don't need, because everything we make … has an adverse impact on the planet."

Moves like that are encouraging to Thompson. “If we can move to products that are long lasting for the consumer, and if at the end of their lifetime the garment can be recycled, that would be the best of all.”