Rolf Halden has been looking at potential toxins through the lens of personal products for years, so it was inevitable that when he switched from glasses to contact lenses he'd wonder what happens at the end of their useful life.

"I was in the bathroom unpacking a lens and I thought, what happens to this?" he says. "What are people doing with it? Occasionally, I would lose a lens and it would end up in the drain on the way to a wastewater treatment plant."

Halden is more familiar with water treatment plants than the average contact lens wearer. He is the director of the Biodesign Institute's Center for Environmental Health Engineering at Arizona State University and he has made a career out of studying potentially harmful chemicals that make their way through the wastewater system.

If you flush it, he and his team likely have examined it. Arizona State is home to an archive of samples from more than 200 wastewater treatment plants across the country, the National Sewage Sludge Repository . His work at ASU and earlier at Johns Hopkins University School of Public Health has examined where mass-produced chemicals end up in the environment.

He and his team study wastewater treatment plants where rakes screen out solids before microorganisms break down and remove organic material in wastewater. A plant is left with sludge that includes both nutrient-rich organic material and man-made contaminants, while the treated water is discharged back into local waterways.

Research from Halden and others has shown that clean water isn't so clean: Halden has found the active ingredient in antimicrobial soaps in estuarine sediments along the East Coast, from treated wastewater discharged into rivers and streams. The Food and Drug Administration banned that ingredient, triclocarban, and other antimicrobial chemicals in 2017, but researchers have also found medicines , illegal drugs and microplastics in treated water.

On land, Halden and his team have found flame retardants, antimicrobial soaps, and opioids in sewage sludge that is deposited on agricultural fields as fertilizer or in landfills, where they can enter the marine ecosystem, invisible but toxic.

Add contact lenses to that list, thanks to his bathroom inspiration.

Halden tasked two protégés, Charlie Rolsky, a doctorate student, and Varun Kelkar from the Biodesign Institute, with examining the issue.

Rolsky and Kelkar isolated fragments they thought were contacts from wastewater sludge, then took them back to the lab. There, using Raman spectroscopy, a process that identifies substances based on their chemical composition, they proved that the fragments were in fact from discarded contacts—confirming that people do discard lenses into drains and toilets. (This was further supported by a survey Rolsky designed that found just under one in five wearers regularly flush their lenses.) Given that about 45 million people in the U.S. wear contact lenses, that could translate to a lot of unnecessary plastic.

Separately, Rolsky and Kelkar put contacts into the anaerobic and aerobic chambers at the treatment plant for 14, 96, and 192 hours to mimic the times various treatment plants expose waste to microbes, then analyzed them to determine the amount of degradation caused by the microbes that break down waste. The longer they were in the tanks, the more the lenses changed as the microbes weakened the plastic polymers. But they didn't fully disappear. "They persisted through the process," Rolsky says. "The take-home was that contacts going through the plant are becoming fragmented and ending up in the biosolids effluent."

The researchers, who presented the results of their work at a recent meeting of the American Chemical Society, concluded that about 44,000 pounds of contact lenses enter wastewater treatment plants annually in the United States. Of that, they say between 24,250 pounds and 27,900 pounds make it into the wastewater sludge applied to land as fertilizer or carted to landfills. "Being around them, having analyzed them, they're so small and flimsy," Rolsky says. "I didn't think it was going to be nearly that big a number. I was surprised at first, but after doing the math, it makes sense."

Halden says lenses in sludge can end up in surface waters. Waste in landfills can leach out when rainwater infiltrates them, he adds. These plastics can then become a drinking water problem and a hazard to marine ecosystems. Or, if they are in sludge used as fertilizer, they could end up in agricultural soil—where little is known about what happens next.

"Contact lenses are one more plastic article that we showed is making the migration into sewage sludge and into landfills, incinerators, or agricultural land and other soils in places where we grow our food," he notes. "This is a common phenomenon of many pollutants we produce in society."

While plastics in water, particularly the oceans, have received much attention in recent years, scientists are turning to look at plastics deposited on the land.

Anderson Abel de Souza Machado, a researcher at the University of Berlin, says scientists are at the beginning of understanding how plastics and microplastics in the soil affect not only animals and microorganisms, but plants. Some studies, he says, show microplastic particles from wastewater treatment plants have are transformed in ways that attract pathogens, spreading them. In a recent study, his team found that fibers, beads, and other plastic particles interfered with plants obtaining necessary water. “The plants showed water stress even though they were in a hydroponic culture,” he adds.

He co-authored a recent paper calling for better understanding of plastics in terrestrial ecosystems, labeling them an emerging threat. "Microplastic and nano plastic pollution might have potentially important, although almost completely neglected, impacts on biodiversity of continental systems," the authors wrote. "There is an urgent need to prioritize research dealing with this topic, and to provide sound information about environmental behavior."

"There is a lot of scaremongering going on,” Machado says. “I think we have to be careful. Much of our knowledge is very, very conceptual. The first papers are just coming out now. There might be effects where we thought there were not. There might be no effects where we thought there were and there might be effects we could not forecast. In terms of data, we still have very, very little.”

Halden says there's a chance that some of the contact lenses remain in the water discharged from the plant. His team did not attempt to measure that amount, something he says is very difficult. "I think it's fair to say a very small portion can migrate through," he adds, "but we are not in a position right now to call out what the quantity is. I'm confident that it's not zero."

A separate study , one of the few examining plastic discharges from wastewater treatment plants, reported that rivers downstream of the six wastewater plants studied had increased levels of microplastics. This "confirms that treated sewage effluent is a key source of microplastics," the authors concluded.

Halden and Rolsky are looking at other possible studies of high-value personal care plastics, things like teeth whitening strips that might slip under the monitoring radar. Wastewater treatment plants, Halden says, are "the undiscovered information superhighway" because of what they reveal about health and behavior and the insight they provide into environmental issues.

Contacts aren't the world's most pressing threat, but they are yet another way for plastics to get into the environment. Halden and Rolsky say the best time to keep plastics out of the wastewater stream is at the beginning.

An easy first step, Halden says, is including language on packaging that lenses should not be flushed or tossed down the drain. (Bausch + Lomb, one of the largest manufacturers of contact lenses in the U.S., instituted a recycling program in 2016, but other companies have yet to follow suit.)

"What surprised me most is there is no information on the packaging of these products because they produce a potential hazard," Halden says. "This is an omission easy to fix."