Tossing used contact lenses down the sink or toilet could be contributing to microplastic pollution in waterways, according to researchers at Arizona State University.

However, they say it's one problem that can be easily addressed by making people aware of some of the staggering numbers related to their disposal.

"We know that between seven and about 15 billion (soft plastic) lenses are being used in the U.S. every year, and the number actually goes up year by year because more and more people use the daily lenses," said Rolf Halden, director of the Center for Environmental Health Engineering at ASU's Biodesign Institute

Some lenses accidentally fall somewhere upon removal or are tossed in the garbage, but many people choose to "just flush them because they have no instructions on what to do with them," he said.

His team's ongoing survey of contact lens wearers in Arizona, which at last count queried about 400 people, found that about one in five of the participants who wear lenses said they flush them down the sink or toilet.

"This is a pretty large number, considering roughly 45 million people in the U.S. alone wear contact lenses," said Charles Rolsky, a PhD student who is working with Halden and a third member of the ASU team, Varun Kelkar.

Leaving treatment plants mostly intact

Lenses that are flushed away travel through the sanitary sewer system and most end up in wastewater treatment plants, and, being heavier than water, they tend to sink to join the sludge.

Mechanical handling was found to have fragmented the outer part of the lenses that the team recovered from the sludge. They also learned that the microbes treatment plants count on to reduce the amount of organic matter did not change the molecular makeup of the polymers found in many contact lenses.

Municipalities can choose to put the sludge into landfill sites, or incinerate it — or take "the most common approach, which is done to about 55 per cent of all the biosolids produced in the United States, which is applying them on land," Halden said.

The plastic can then enter the food chain through earthworms in the soil, or it can get into rivers, lakes and oceans through runoff when it rains. Some of the plastic takes a more direct route to waterways when there are overflows at treatment plants.

Plastic waste is considered a microplastic after it breaks down into tiny shards less than five millimetres (5,000 microns) in length.

The team estimates that anywhere from six to 10 metric tons of plastic lenses end up in wastewater in the U.S. each year. ​

Further, the plastics used in contact lenses are different from other plastic waste, such as polypropylene, which can be found in everything from car batteries to textiles.

The researchers say the plastics used for contact lenses can likely soak up contaminants from the sludge at wastewater treatment plants. (Brian Snyder/Reuters)

Contact lenses are frequently made with a combination of poly(methyl methacrylate), silicones and fluoropolymers to create a softer material that allows oxygen to pass through the lens to the eye.

"The plastics may have the capacity of soaking up contaminants, and so the plastic shards ... they'll likely be loaded with toxic chemicals, like heavy metals, PCBs [polychlorinated biphenyls] and other things," Halden said.

He said he thinks the research can help "start a dialogue with the producers of contact lenses and to work with them to take better care of the material flow" — and to encourage them to include instructions on their packaging to let consumers know to toss them in the trash.