Per- and polyfluoroalkyl substances (PFAS) are a group of human-made chemicals known as “forever chemicals” because they linger in the environment—and in human bodies.

Exposure to high levels of PFAS has been linked to challenges with metabolic2 and thyroid health, adverse birth outcomes3, and in some extreme cases, higher incidents of cancer. PFAS are less than 100 years old (we first started using them to make water- and oil-resistant products in the 1940s), so there are still a lot of unanswered questions about how prevalent they actually are in our water supply.

Up until this point, most PFAS testing has been done at water treatment plants—not kitchen sinks themselves. In-home testing can be logistically difficult, but it’s something that the USGS wanted to prioritize in this round of research because, as environmental organic chemist Kelly L. Smalling, MSPH, explains, “That’s where exposure actually happens.”

“Most of the information available on PFAS is at the drinking water treatment plant or in the surface water and groundwater wells that actually supply them. But then there are miles of infrastructure between the treatment plant and your home,” Smalling, who worked on this new study, says on a call with mindbodygreen. “We don’t really know how or if it changes as it moves through the distribution pipeline.”

To find out how many PFAS are actually present at the tap level, the USGS worked with a network of volunteers to collect water samples from 716 residences across the U.S. This included a mix of rural and urban residences and public water supplies and private wells. Volunteers sent samples to laboratories to be tested for 32 types of PFAS.

While the USGS has conducted in-home PFAS testing before, it’s been on a much smaller scale. This new research aimed to paint a more comprehensive picture of how many chemicals are actually present in our drinking water and how it varies across the country.