Almost 50% of tap water in the US is contaminated with "forever chemicals," a new study from the US Geological Survey suggested. However, because the researchers were unable to test for all of these per- and polyfluorinated alkyl substances, or PFAS, chemicals that are considered hazardous to human health, the number of people drinking contaminated water may be even higher than what the study found.

According to the National Institute of Health, there are over 12,000 different types of PFAS, but this study only looked at 32 of them.

PFAS are a class of synthetic chemicals that are ubiquitous in the environment and the human body.  PFAS exposure is linked to cancer, according to the US Environmental Protection Agency, obesity, thyroid disease, high cholesterol, decreased fertility, liver damage, and hormone suppression.

Based on the most recent science, the EPA issued health advisories in June 2022, stating that the chemicals are much more hazardous to human health than scientists initially thought, and are likely to be more dangerous even at levels thousands of times lower than previously thought.

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That said, health effects from the chemicals can be difficult to specify, in part because people may be exposed in different ways and at different stages of development and because there are so many types of PFAS chemicals with types and uses that have changed over time.

Most people in the United States have been exposed to some PFAS, and some may be more vulnerable, such as industrial workers involved in the production of PFAS and people who live near those facilities. There is ongoing research to determine how different levels of PFAS chemical exposure may result in different health effects.

Experts explain it is critical for people to understand their risk of exposure to tap water. If your tap water is contaminated, water filters can help, and there are efforts underway to regulate some PFAS chemicals in US drinking water.

Where PFAS concentrations are higher

According to the authors of the study, which was published Wednesday in the journal Environmental International, there was previously little information on how much PFAS chemicals were present in residential tap water. They also claimed that this is the most thorough study to date, encompassing both private wells and public water sources.

Between 2016 and 2021, the scientists collected water samples directly from taps at 716 locations, 269 from private wells, and 447 from public sources. According to their findings, at least one PFAS chemical would be detected in 45% of US drinking water samples.

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Most of the contamination came from water sources near urban areas and areas that generated PFAS, such as manufacturing that uses chemicals in its products or waste collection sites.

According to the study, the Great Plains, Great Lakes, Eastern Seaboard, and Central/Southern California had the highest concentrations of PFAS in drinking water. Concentrations were similar between private wells and public supplies.