If you throw out your contact lenses every day or so, you’re not alone — more than 45 million people in the United States wear contacts, and many of them use disposable versions of the little plastic hemispheres.

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 who were randomly recruited in an online survey flushed used contacts down the toilet or washed them down the sink, rather than putting them in the garbage.

When the lenses make their way to a wastewater treatment facility, 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 swirling in the ocean.

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Filters keep some nonbiological waste out of wastewater treatment plants, said Rolf Halden, the director of the Center for Environmental Health Engineering at Arizona State University, and Charles Rolsky, a graduate student and the study’s lead author. (Dr. Halden uses contact lenses; Mr. 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.