When a pair of contact lenses reach the end of their short life span, it may be tempting to dump them—and the liquid they’re stored in—down the bathroom sink drain. As The Atlantic points out, though, this is bad for the environment.

However small and thin they may be, contact lenses can contribute to microplastic pollution in waterways when they’re not disposed of properly, according to a new study presented at the recent American Chemical Society national meeting in Boston. The study surveyed contact lens users and non-users, and found that 19 percent of users flush the lenses down the toilet or sink drain instead of placing them in the trash. That translates to about 3 billion contact lenses per year, Rolf Halden, a researcher at Arizona State University and one of the study's authors, told The Atlantic.

Halden said he was inspired to look into the issue out of personal interest—he, too, wears contact lenses—and because he couldn’t find any studies on what happens to lenses after they’re flushed down the drain. Halden and his team discovered that the lenses end up in wastewater, where they can sink to the bottom because they are denser than water. There, they could endanger aquatic life, especially bottom feeders that may ingest the particles.

Researchers also found that microbes in wastewater treatment plant systems can degrade the structure of the lenses themselves and break them into tinier pieces. In the larger environment, those bits could be consumed by every organism in the local food chain. In addition, some of the flushed lenses are turned into a treated sewage sludge that is ultimately used to fertilize crops, so the waste could end up in our soil and affect creatures like earthworms. The extent to which this affects humans' food supply is not currently known.

"Ultimately, we hope that manufacturers will conduct more research on how the lenses impact aquatic life and how fast the lenses degrade in a marine environment,” Halden says in a statement.

So the next time you’re done using your contact lenses, think of the fish and worms, and throw them in the trash instead.

[h/t The Atlantic]