Flushing contact lenses down the sink? You’re part of a growing problem.

Ever take out your contact lenses and drop them down the sink?

Those tiny lenses are contributing to a much larger plastics problem, say researchers.

Almost one-fifth of disposable contact lens wearers flush them down sinks or toilets, amounting to 10 metric tons of microplastics flowing each year into U.S. wastewater plants and ultimately waterways, posing a problem for fish and aquatic life, a team of researchers said Monday.

The inspiration for the study was simple, said one of the scientists, Rolf Halden: "I had worn glasses and contact lenses for most of my adult life. But I started to wonder, has anyone done research on what happens to these plastic lenses?" The researchers couldn't find any other studies on how wearers got rid of the soft, disposable lenses that are ever-increasing in popularity.

Halden, a professor at the Biodesign Institute at Arizona State University, and two other researchers presented their findings in Boston at the 256th national meeting of the American Chemical Society.

It's estimated that up to 45 million Americans use contact lenses.  So the researchers surveyed 400 users online and found that 19 percent of those said they simply flush the used lenses down sinks or toilets — even those who said they were environmentally conscious.  That may be because the lenses themselves have a different, "watery" feel than most other plastics and users assume they are made to be flushed.

The lenses are made with a combination of chemical compounds that differ from those used in other plastics, such as water bottles.  Instead, the compounds used in lenses create a softer plastic that allows oxygen to pass through to the eye.  But the researchers found the lenses did not fully degrade — though the polymers did break down rather quickly into even smaller plastics, known as microplastics, when they were in a "real world"  treatment facility.

After their trip through treatment tanks, the lenses or bits of lenses ended up being discharged into waterways or into sewage sludge, which often gets spread on land.   In both cases, the plastics can end up in the food chain as they are ingested by aquatic life or even worms.

Halden also said the team discovered the lenses pose a bigger threat by absorbing atmospheric and water pollutants in "large quantities" comparative to their size.  Contact lenses tend to be denser than water, so they sink and could end up being ingested by bottom feeding aquatic life.

"It seems like such a small problem, but they come by the billions," Halden said of the lenses — and that doesn't include their packaging.

"As chemists, this should encourage us to find a better balance between the useful life and the polluting afterlife of the products," Halden said, noting the lenses are just part of the bigger general problem of plastics made from fossil fuels.

He referred to those plastics as "old-style chemistry"said "smarter polymers" are needed.

The scientists were surprised at the amount of media attention their presentation received.

"We really sort of hit a nerve with this," Halden said.

Halden urged users to just throw their lenses in the trash, where they would be contained in landfills.