If you throw out your contact lenses every day or so, you’re not alone — more than 45 million people in the U.S. wear contacts, and many are disposable. But if they are not tossed out correctly, contact lenses may have a dark side.

Research presented Sunday at the American Chemical Society’s meeting in Boston showed that 20 percent of more than 400 contact wearers randomly recruited in an online survey flushed contacts down the toilet or washed them down the sink rather than putting them in the trash.

When the lenses make their way to wastewater plants, they do not biodegrade easily, the researchers report, and they may fragment and make their way into surface water. There, they can cause environmental damage and may add to the growing problem of microplastic pollution. A 2015 study found that there were 93,000 to 236,000 metric tons of microplastic in the ocean.

Filters keep some nonbiological waste out of waste­water treatment plants, said Rolf Halden, director of the Center for Environmental Health Engineering at Arizona State, and Charles Rolsky, a graduate student and the study’s lead author. (Halden uses contact lenses; Rolsky wears glasses.) But contacts are so flexible that they can fold up and make their way through. The researchers interviewed workers at such facilities, who confirmed that they had spotted lenses in the waste.

Next, the team submerged contacts in chambers where bacteria are used to break down biological waste at a treatment plant. They found that even after seven days of exposure, the lenses appeared intact, though lab analysis detected small changes.

“These are medical devices — you would not expect them to be super-biodegradable,” Halden said. “Good for the contact lens wearer during use, not so good when the things get out into the environment.”

Then, going through about 9 pounds of treated waste, Rolsky and a colleague found two fragments of contact lens, implying that while microorganisms might not make much of a mark, physical processing might break them into pieces.

Tiny bits of plastic from many sources have been spotted in the oceans and other water, where they may be ingested by fish, corals and other animals. The fragments can carry high loads of pollutants absorbed from their surroundings, so the organisms get a dose of these substances as well.

Treated waste is often spread on fields. If fragments of contacts are in the mixture, they or the substances they’ve picked up may be washed by rain into surface water, the researchers conjecture. They estimate that billions of contact lenses — weighing at least 22 metric tons — may be flushed in the U.S. each year.

Contact lens packages don’t tell users how to dispose of them, said Halden, who suggested that companies should add labels recommending that contacts be put in the garbage rather than washed down the drain. He pointed out that some manufacturers have already begun recycling programs to reclaim the plastic.

Rolsky said the findings had given him an additional incentive to stick with wearing glasses. “This just gives me a reason,” he said with a laugh, “to solidify that for the rest of time.”