As the world grapples with climate change, population growth and dwindling supplies of fresh watermore and more people will rely on treated wastewater to sustain their daily lives.

But wastewater, even after treatment, contains high levels of harmful “forever chemicals” that are already contaminating the drinking water of millions of Americans, researchers said in study published on Monday which analyzed wastewater samples across the country.

The study, led by researchers from Harvard University and New York University, found elevated levels of six types of chemicals known as PFAS, or per- and polyfluoroalkyl substances, in the samples. Chemicals that were associated with cancer and other diseasesthey are known as eternal chemicals because they do not break down in the environment. Last year, the Environmental Protection Agency began regulating PFAS in drinking water.

The researchers found that the samples contained even higher amounts of organofluorines, a broader group of chemicals that includes PFAS and is used in pharmaceuticals, refrigerants and non-stick coatings. Most of these chemicals are not regulated, and the health consequences of exposure to many of them are still unknown.

“What are all these other dates? Are these other PFASs that we’re not measuring that the industry has turned to?” said Bridger Ruyle, assistant professor of environmental engineering at New York University, who led the research. “What does that mean for exposure?”

The study, published in the Proceedings of the National Academy of Sciences, found that wastewater treatment plants do not effectively remove these compounds from wastewater. Namely, in most locations, PFAS in wastewater became more concentrated after treatment, the researchers found.

Contamination is of particular concern, the researchers said, given that water scarcity in many regions of the United States means that wastewater is reused or discharged into rivers and lakes. And if that wastewater is not sufficiently diluted before re-entering the drinking water supply, a growing concern as water flows decline due to overuse and climate change, “you have a contamination problem,” Professor Ruyle said.

About 50 percent of the country’s drinking water supply is located downstream from one or more wastewater sites, he added. The study used modeling to show that PFAS from wastewater is already contaminating the drinking water of up to 23 million people in the United States.

The results “underscore the importance of further suppression of liquid sources of PFAS,” the researchers concluded.

A new study highlights how widespread contamination is complicating efforts to reuse wastewater, which includes sewage from households as well as polluted water from businesses and factories. Sludge left over from wastewater treatment is also used to fertilize agricultural land across the country, i PFAS contamination of that sludge also raises concerns about the practice.