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

Exposure to high levels of PFAS has been linked to problems with metabolic 2 and thyroid health, adverse birth outcome3, and in some extreme cases – a higher number of cancer cases. 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 many unanswered questions about how prevalent they really are in our water supply.

Until now, most PFAS testing has been done on water treatment plants, not on the kitchen sinks themselves. Testing at home can be logistically challenging, but the USGS wanted to prioritize it in this round of research because, as organic chemist Kelly L. Smalling, MSPH, explains, “that’s where the impact really happens.”

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

To find out how much PFAS is actually present at the tap level, the USGS worked with a network of volunteers to collect water samples from 716 homes across the United States. This included a mix of rural and urban homes, public water sources and private wells. Volunteers sent samples to a lab to be tested for 32 types of PFAS.

While the USGS has already conducted home testing for PFAS, it has been on a much smaller scale. This new study aimed to paint a more complete picture of how many chemicals are actually in our drinking water and how they vary by country.