Don’t flush your contact lenses | Science News for Students

Don’t flush your contact lenses

The plastics in them can pollute water and be mistaken for food by aquatic animals
Oct 9, 2018 — 6:45 am EST
a photo of a young woman putting in a contact lens, reflected in a mirror

One in five people who wear contact lenses flush their used eyewear down the sink or toilet. That plastic pollutes the environment and can harm wildlife.


If you wear contact lenses, you might not know the best way to discard old ones. Hint: Washing them down the sink or flushing them down the toilet are not the way to go. Yet one in five people who wear contacts do just that. The bad news: The plastic in their lenses can linger, polluting both water and land.

Researchers quantified the problem, this past August, at the American Chemical Society meeting in Boston, Mass. Rolf Halden was one of them. This engineer at Arizona State University in Tempe also described his team’s findings for reporters at a press conference from the meeting.

Halden studies ways to reduce pollution’s effect on the environment. Many people need contact lenses every day for work and play. Until now, he notes, scientists hadn’t studied what happened to these soft plastic disks after use.

a photo of two contact lens fragments
When flushed, contact-lens fragments end up in sewage sludge. These plastics don’t fully degrade.
Charles Rolsky

But having worn contact lenses for many years, he could relate to the issue. One day, he thought about what happens to all of those disposable lenses after people are done with them. He and his graduate students, Charles Rolsky and Varun Kelkar, designed some tests to find out.

They started by creating an online survey. More than 400 contact lens wearers took part. The questions asked how many disposed of their lenses improperly. About 20 percent — one in five — sent their used contacts down a sink drain or toilet.

Assuming all U.S. contact-lens wearers do that at the same rate, the researchers then calculated how much plastic would be flushed away each year. Their estimate: 6 to 10 metric tons! That’s about the weight of two to three adult African forest elephants.

Contact lenses are a tiny part of the world’s plastic pollution. A total of 8 million metric tons of plastic end up in the world’s oceans each year. But the unique chemistry of the plastic used in contact lenses could make them a big concern, says Terry Collins. He’s director of the Institute of Green Science at Carnegie Mellon University in Pittsburgh, Pa. He wasn’t involved in the study.

A new form of pollution

Wastewater treatment plants clean the water that washes down drains or that exits toilets. They separate out raw sewage (things like poop as well as other items, such as twigs and garbage) and use microbes to clean the rest of the water. Then they release the water into lakes and rivers. In the laboratory, Halden’s team probed what would happen to contact lenses at a treatment plant.

Contact lenses are made from soft plastics. They are a type of long chain-like molecules known as polymers (PAHL-ih-murs). Each link in the chain holds onto to its neighbors by chemical bonds.

To find out how the wastewater treatment process affects those bonds, the researchers exposed contact lenses to the microbes found in water-treatment plants. These microbes broke some of the chemical bonds. This caused the plastics to begin to fall apart. But they didn’t fully degrade. Instead, they created a lot of tiny bits of debris. Scientists refer to those bits as microplastics. And those would wash out of the water-treatment plant, along with the “cleaned” water.

“Contact lenses are a very new form of plastic pollution,” said Halden. He hopes that others will study these plastics, too.

Environmental scientists have been finding microplastics everywhere, from the ocean floor to mountaintops. And that concerns them, says Collins, because the chemical bonds that remain in these smaller plastics are “almost indestructible.” That could allow the pollution to persist for a long time.

Wildlife can view this pollution as food

Collins, Halden and others worry that these small plastic bits will cause trouble in the food chain. In water, the plastics from contact lenses sink. Animals could view these tiny bits as food. But because the plastic won’t provide them nutrients, this could threaten the health — certainly the growth — of animals who dined on it.

And that’s already happening. Many studies have shown that corals, larval fish and shellfish are mistaking microplastics for food. Any animal that eats these polluted prey will get those plastics into its body, too. Over time, they risk accumulating ever higher levels of plastic in their bodies. The pollution can reach people, too. It has already shown up in bottled water, sea salt and fish sold for human consumption.

Scientists don’t yet know whether a build-up of microplastics will harm most aquatic animals and people. But its presence is not a good sign. Among other problems with these plastic bits: They tend to accumulate many of toxic water pollutants. These can include pesticides, fossil-fuel wastes and polychlorinated biphenyls. Plastic bits that carry these pollutants now risk bringing them into the diet of animals — including people.

So what should people do with their used contact lenses? Throw them in the garbage rather than flushing them, Halden says. Urging contact lens makers to recycle used lenses might help, too. Chemists might also try to design new plastics, he says. Their goal: plastics tough enough to do their job, but that degrade when no longer needed.

People need to be more aware of plastics, Collins says. “We throw plastic away without much thought. Any solution [to this pollution],” he says, “must involve changing the way we think about — and use — plastic.”

Power Words

(more about Power Words)

aquatic     An adjective that refers to water.

bond     (in chemistry) A semi-permanent attachment between atoms — or groups of atoms — in a molecule. It’s formed by an attractive force between the participating atoms. Once bonded, the atoms will work as a unit. To separate the component atoms, energy must be supplied to the molecule as heat or some other type of radiation.

chemical     A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

chemical bonds     Attractive forces between atoms that are strong enough to make the linked elements function as a single unit. Some of the attractive forces are weak, some are very strong. All bonds appear to link atoms through a sharing of — or an attempt to share — electrons.

chemistry     The field of science that deals with the composition, structure and properties of substances and how they interact. Scientists use this knowledge to study unfamiliar substances, to reproduce large quantities of useful substances or to design and create new and useful substances.

coral     Marine animals that often produce a hard and stony exoskeleton and tend to live on reefs (the exoskeletons of dead ancestor corals).

debris     Scattered fragments, typically of trash or of something that has been destroyed. Space debris, for instance, includes the wreckage of defunct satellites and spacecraft.

degrade     To break down into smaller, simpler materials — as when wood rots or as a flag that’s left outdoors in the weather will fray, fade and fall apart. (in chemistry) To break down a compound into smaller components.

diet     The foods and liquids ingested by an animal to provide the nutrition it needs to grow and maintain health. (verb) To adopt a specific food-intake plan for the purpose of controlling body weight.

engineer     A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.

environment     The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of components in some electronics system or product).

fossil fuels     Any fuels — such as coal, petroleum (crude oil) or natural gas — that have developed within the Earth over millions of years from the decayed remains of bacteria, plants or animals.

graduate student     Someone working toward an advanced degree by taking classes and performing research. This work is done after the student has already graduated from college (usually with a four-year degree).

green     (in chemistry and environmental science) An adjective to describe products and processes that will pose little or no harm to living things or the environment.

lens     (in biology) A transparent part of the eye behind the colored iris that focuses incoming light onto the light-absorbing membrane at the back of the eyeball. (in optics) A curved piece of transparent material (such as glass) that bends incoming light in such a way as to focus it at a particular point in space. Or something, such as gravity, that can mimic some of the light bending attributes of a physical lens.

link     A connection between two people or things.

mass     A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

microbe     Short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.

microplastic     A small piece of plastic, 5 millimeters (0.2 inch) or smaller in size. Microplastics may have been produced at that small size, or their size may be the result of the breakdown of water bottles, plastic bags or other things that started out larger.

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

nutrient     A vitamin, mineral, fat, carbohydrate or protein that a plant, animal or other organism requires as part of its food in order to survive.

online     (n.) On the internet. (adj.) A term for what can be found or accessed on the internet.

pesticide     A chemical or mix of compounds used to kill insects, rodents or other organisms harmful to cultivated plants, pets or livestock; or unwanted organisms that infest homes, offices, farm buildings and other protected structures.

plastic     Any of a series of materials that are easily deformable; or synthetic materials that have been made from polymers (long strings of some building-block molecule) that tend to be lightweight, inexpensive and resistant to degradation.

pollutant     A substance that taints something — such as the air, water, our bodies or products. Some pollutants are chemicals, such as pesticides. Others may be radiation, including excess heat or light. Even weeds and other invasive species can be considered a type of biological pollution.

polychlorinated biphenyls     (or PCBs ) A family of 209 chlorine-based compounds with a similar chemical structure. They were used for many decades as a nonflammable fluid for insulating electrical transformers. Some companies also used them in making certain hydraulic fluids, lubricants and inks. Their production has been banned in North America and many countries throughout the world since around 1980.

polymer     A substance made from long chains of repeating groups of atoms. Manufactured polymers include nylon, polyvinyl chloride (better known as PVC) and many types of plastics. Natural polymers include rubber, silk and cellulose (found in plants and used to make paper, for example).

prey     (n.) Animal species eaten by others. (v.) To attack and eat another species.

recycle     To find new uses for something — or parts of something — that might otherwise be discarded, or treated as waste.

risk     The chance or mathematical likelihood that some bad thing might happen. For instance, exposure to radiation poses a risk of cancer. Or the hazard — or peril — itself. (For instance: Among cancer risks that the people faced were radiation and drinking water tainted with arsenic.)

sea     An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.

sewage     Wastes — primarily urine and feces — that are mixed with water and flushed away from homes through a system of pipes for disposal in the environment (sometimes after being treated in a big water-treatment plant).

survey     (v.) To ask questions that glean data on the opinions, practices (such as dining or sleeping habits), knowledge or skills of a broad range of people. Researchers select the number and types of people questioned in hopes that the answers these individuals give will be representative of others who are their age, belong to the same ethnic group or live in the same region. (n.) The list of questions that will be offered to glean those data.

toxic     Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.

unique     Something that is unlike anything else; the only one of its kind.

waste     Any materials that are left over from biological or other systems that have no value, so they can be disposed of as trash or recycled for some new use.

wastewater     Any water that has been used for some purpose (such as cleaning) and no longer is clean or safe enough for use without some type of treatment. Examples include the water that goes down the kitchen sink or bathtub or water that has been used in manufacturing some product, such as a dyed fabric.


Meeting:​​ C. Rolsky et al. Chemical and physical changes in a variety of contact lenses during the wastewater treatment process. American Chemical Society meeting, Boston, August 19, 2018.