It’s getting cold. Time to break out the fleece. Warm and light, it even comes in a nice shade of green: a lot of the material used to make fleece comes from recycled plastic bottles – better in your sweater than in a landfill.

Green that is, until you run it through the washing machine. That’s when thousands of plastic microfibers get flushed into the sewer system and on to some stream, lake, river and ocean.

In a world where greenhouse gas threatens to bake the planet, this might not seem so worrisome – except that scientists are starting to suspect it is. Fleece is polyester, which isn’t biodegradable. It’s made from oil, and like most plastic it’s almost immune to the elements. These tiny bits of plastic end up being consumed by small fish and filter-feeders such as oysters and clams. The stuff can then make it up the food chain and, potentially, into people. Scientists are still trying to figure out how much of a risk this is to human health.

There’s an added concern. PCBs and other contaminants tend to stick to microfibers. For marine life, consuming plastic is bad enough; consuming plastic glued to toxins has to be worse.

This microscopic junk is almost everywhere; during a single washing one sweater will shed almost 2,000 fibers. One team of researchers gathered sand from 18 beaches on six continents. They didn’t find one that was free of the microfibers. The highest concentrations were found on beaches near cities – in other words, where a lot of people wash a lot of clothing.

Of course, the earth is choked with plastic. There’s even a place in the Pacific Ocean where waste gathers in circulating currents to create a gigantic patch of garbage, much of it made up of plastic refuse.

Some modest progress is being made to get plastic out of our water. Another major source of microscopic plastic waste is what’s known as microbeads – those little granules in your body scrub, soap and toothpaste that exfoliate your skin or help scour crud off your teeth. These tiny bits of plastic make the same journey as microfibers, passing through sewage treatment systems and into rivers, lakes and streams.

Microbeads, which often are heavier than water, are so prevalent that they’re covering the bottoms of the Great Lakes in a layer of plastic sediment. Earlier this month California adopted a law banning use of microbeads starting in 2020. At least six other states have passed bans, though most states allow a loophole for microbeads that are supposed to be biodegradable. But California, the most populous state, is banning all microbeads (as is New Jersey), so consumer-goods makers such as Procter & Gamble and Johnson & Johnson are either considering or planning to remove them from their products.

Don’t expect states to start banning fleece, though. That would be radical (though wool producers might cheer). Instead, there are several possible ways to limit microfiber pollution.

One might be to produce fleece that sheds less. Patagonia, one of the best-known purveyors of fleece garments, is funding research and is working with a company called Tersus Solutions to develop waterless manufacturing systems. Processing textiles uses and contaminates staggering amounts of water.

Another way would be to outfit washing machines with filters to capture the fibers before they head down the drain. I checked with the Association of Home Appliance Manufacturers, and the group is monitoring the research but that’s about it. Anyway, this kind of solution would put the onus on millions of consumers, who would have to clean and maintain their washing-machine filtration systems.

Some researchers have suggested that a better place to capture the fibers would be at sewage treatment plants. I checked with the American Waste Water Association, but the group is mainly focused on energy efficiency and reduced water use. Another group that deals with sewage issues, the Water Environment Federation, says its big challenge at the moment is figuring out how to deal with disposable wipes.

Here’s one thing to make you feel better about fleece. Natural fibers like cotton and wool have their environmental downsides too. Yes, the fibers they shed degrade naturally. But growing cotton requires vast amounts of land, water, pesticides and fertilizer, while raising sheep has caused shocking degradation of millions of acres in countries such as South Africa and Australia.

So maybe the thing to do is keep your fleece. Only don’t wash it until it’s really, really dirty.