(CNN) Millions of Americans may be drinking water with unsafe levels of industrial chemicals, according to a study published Tuesday in the journal Environmental Science & Technology Letters. These chemicals, known as polyfluoroalkyl and perfluoroalkyl substances or PFASs, have been linked to high cholesterol, obesity, hormone suppression -- and even cancer.

Introduced more than 60 years ago, PFASs are a category of man-made chemicals that degrade very slowly, if at all, in the environment.

"I do think that Americans should be concerned about these chemicals," said Susan M. Pinney, a professor in the department of environmental health at University of Cincinnati. Pinney, who did not participate in the new research, explained that not enough time has elapsed to understand all the long-term health effects of these toxins.

Persistent chemicals

"PFASs are organic compounds that are really useful," said Xindi Hu, lead author of the new study and a doctoral student at Harvard T.H. Chan School of Public Health. This usefulness means these chemicals are used to make such items as food packaging materials (such as pizza boxes and popcorn bags), fabrics, nonstick cooking pans and firefighting foams.

As a result of their ubiquity, the chemicals migrate into air, household dust, food, soil and ground and surface water, and they eventually make their way into drinking water.

The problem with PFASs is that they remain in your body for a long time. Though other chemicals can be excreted within hours, it takes about 3½ years for your body to get rid of just half of whatever amount you ingest, Pinney explained, speaking of one particular PFAS she has studied. If you are exposed day after day, they will accumulate in your body.

"We know this chemical gets stored in the blood serum, the liver and some other organs," Pinney said. While the health effects may not be "huge," subtle changes in cholesterol levels and timing of puberty may have important health consequences if they become prevalent in the population as a whole. And, she says, not all the physical effects are currently known.

But PFASs seem to be everywhere. They are found "in wildlife and human tissue and bodily fluids all over the globe," explained Arlene Blum, a co-author of the new study and executive director of Green Science Policy Institute. A chemist, she spearheaded a 2015 statement signed by 200 international scientists to urge restricted use of PFASs.

Public water supply

For their new study, Hu, Blum and their colleagues examined more than 36,000 water samples collected by the Environmental Protection Agency between 2013 and 2015.

The researchers discovered that 66 public water supplies serving 6 million Americans had at least one water sample that measured at or above the EPA recommended safety limit of 70 parts per trillion for perfluorooctanesulfonic acid and perfluorooctanoic acid, two types of PFASs. Newark, Delaware, and Warminster, Pennsylvania, showed particularly high concentration levels.

Though the EPA (PDF) declined to comment specifically on this research, the agency noted that the primary manufacturer of perfluorooctanesulfonic acid voluntarily phased out the chemical from production in the U.S. between 2000 and 2002. Four years later, eight major companies agreed to cease their global production of perfluorooctanoic acid and related chemicals, although a few ongoing uses remain.

Still, the study showed that 16.5 million Americans have one of six types of PFASs in their drinking water at levels at or above the maximum EPA limit. Overall, the highest levels were in watersheds near industrial sites, military bases and wastewater treatment plants.

Though 194 public water supplies with higher-than-recommended chemical concentrations are located in 33 states, three-quarters of the toxic water supplies are in just 13 states: California, New Jersey, North Carolina, Alabama, Florida, Pennsylvania, Ohio, New York, Georgia, Minnesota, Arizona, Massachusetts and Illinois.

Related research

A second study from Harvard T.H. Chan School of Public Health, this one appearing in Environmental Health Perspectives, suggested that these chemicals may be disrupting children's immune health.

Researchers led by Dr. Philippe Grandjean, adjunct professor of environmental health, examined a group of about 600 teens from the Faroe Islands. Those exposed to PFASs at a young age had lower-than-expected levels of antibodies against diphtheria and tetanus, for which they had been immunized.

"Others have seen the same effect for measles and influenza," Grandjean said, noting that such results suggest that PFASs, which are known to interfere with immune function, may be involved in reducing the effectiveness of vaccines in children. "To what extent [PFASs] interfere with other immune functions is unclear, e.g. allergy or autoimmunity, or response to cancer cells," he explained in an email, noting that some PFASs are carcinogenic, though scientists do not understand exactly how they contribute to cancer.

As with any environmental chemical, it takes a long time to understand its human health effects, Pinney said, explaining that the most vulnerable periods for exposure are in utero and during infancy.

The good news is that in the case of perfluorooctanoic acid, one of the the most extensively produced and studied chemicals within the PFAS family of 268 compounds , the concentration levels measured in people's blood are "clearly coming down" within the general population, Pinney said. "This is clearly due to regulations that have reduced industrial use and industrial emission."

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Because perfluorooctanoic acid has been studied extensively, she says, its negative effects became known, and the industry stopped using it. But whenever you ban a particular chemical, you often get its "chemical cousin," explained Blum, who wants restrictions placed on all PFAS chemicals. In fact, Blum inspired two other recent research efforts in addition to the two studies presented here to flesh out the story of their ill effects.

"Our position is, given that these chemicals never break down in our environment, they should only be used when needed," Blum said. "What we consider the most harmful chemicals can be reduced by 50%, and that would be a huge benefit to our health."