Nearly half of the tap water in the U.S. is contaminated with “forever chemicals,” a new study from the U.S. Geological Survey determined. These chemicals, called per- and polyfluorinated alkyl substances or “PFAS,” are a group of synthetic chemicals that don’t break down in the human body and the environment. They’re used in kitchenware, packaging, and clothing to make products greaseproof and waterproof. Last year, the chemicals were found to be far more hazardous than previously thought—even in low levels. The new study found that 45 percent of U.S. drinking water contains these substances, but the real number is likely much higher considering the researchers weren’t able to test for all of these compounds. Much is left to learn about “forever chemicals,” but they appear to be increasingly common as research continues. “There’s been almost no place scientists have looked where they have not found PFAS,” Dr. Jamie DeWitt of East Carolina University told CNN.