Rebecca Rosenberg typically throws away her contacts in the trash, but occasionally disposes of them in the toilet “either because my trash was empty and I didn’t want them to stick to the bottom, or when I’m really mad at them for hurting my eye, I angrily flush to say goodbye,” she says. “Sometimes the toilet is also just closer.”

And Celia Eckert doesn’t dispose of them in the toilet, but does flush them down the sink, “usually because they accidentally get in there,” she says. She rinses them down the drain because “they’re natural-ish … They’re in your body.”

Approximately 14 billion contact lenses get discarded every year in the United States, 3 billion of which are flushed down the toilet or sink, says Rolf Halden, the director of the Biodesign Center for Environmental Health Engineering at Arizona State University and one of the authors of the new study. The issue with flushing them, he says, is that the contacts don’t decompose, and as a result, nearly 50 tons of contacts could end up in American water supplies each year.

His study, one of the first of its kind, surveyed 409 contacts wearers on their disposal habits and found each year that approximately 28,000 pounds of lenses end up being turned into treated sewage sludge that’s used as crop fertilizer. That means that tiny pieces of transparent plastic, known as microplastic, are sinking into the soil, which could then impact small animals, insects, and even end up in groundwater sources that eventually flow into the ocean.

So far, studies on the effects of microplastics have primarily focused on oceans, but researchers are beginning to study the impacts on land as well, and it’s not pretty. The environmental and health effects of microplastics are “a big topic that’s being focused on, and is kind of the hot question right now: How bad is the problem actually?” says Lisa Watkins, a graduate student and the lead of Cornell University’s Soil and Water Lab microplastics-research team. The answer is unclear. “We find that earthworms carry that plastic [from the sewage sludge] into their burrows,” Watkins says. “But there haven’t been that many studies on what happens to plastics on land.”

Halden acknowledges that there’s a lack of concrete evidence for the lasting effects. “If [microplastics] get out in sewage sludge and end up in the environment, they might pose a risk to aquatic organisms—think mussels and fish,” he says. “We don’t know if these plastics go up the food chain. Earthworms take them up, birds could eat them, and what happens when a bird ingests the contact lens? Does it disrupt the G.I. tract? These are all unanswered questions.”

Despite the need for more evidence, Nick Mallos, the director of the Trash Free Seas program at the Ocean Conservancy, remains concerned about how these improperly disposed-of contact lenses could be affecting waterways. “Any amount of plastic that is getting into the environment has a potential impact,” he says. “It’s really important that we not get fixated on the magnitude of inputs. Certainly, when we think of solutions and how we prioritize interventions, it’s important to think about those trade-offs. But for something like this, we know the source, and it really is a simple fix.”