Research has found treated waste water has been shown unable to reliably filter out contact lenses, so those who flush their lenses down the toilet are contributing microplastic pollution, according to The New York Times.

"These are medical devices — you would not expect them to be super-biodegradable," the Director of the Center for Environmental Health Engineering at Arizona State University, Rolf Halden, told the Times. "Good for the contact lens wearer during use, not so good when the things get out into the environment."

The American Chemical Society research presented Sunday in Boston found 20 percent of 400 contact lens users flush old contact lenses down the toilet or in the sink.

Even if the whole contact lens does not escape through waste water filters, the fragments of them can be dangerous, too, contaminating the environment.

The research estimates billions of contact lenses may be flushed every year, some 22 metric tons, according to the Times.

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