Guardian analysis of water samples from around the United States shows that the type of water testing relied on by the US Environmental Protection Agency (EPA) is so limited in scope that it is probably missing significant levels of PFAS pollutants. The undercount leaves regulators with an incomplete picture of the extent of PFAS contamination and reveals how millions of people may be facing an unknown health risk in their drinking water. The analysis checked water samples from PFAS hot spots around the country with two types of tests: an EPA-developed method that detects 30 types of the approximately 9,000 PFAS compounds, and another that checks for a marker of all PFAS. Seven of the nine samples collected showed higher levels of PFAS in water using the test that identifies markers for PFAS, than levels found when the water was tested using the EPA method and at concentrations as much as 24 times greater. PFAS ... are often called "forever chemicals" because they don't fully break down, accumulating in the environment. Some are toxic at very low levels and have been linked to cancer, birth defects, kidney disease, liver problems, decreased immunity and other serious health issues. The limitations of the test used by state and federal regulators, which is called the EPA 537 method, virtually guarantees regulators will never have a full picture of contamination levels as industry churns out new compounds much faster than researchers can develop the science to measure them.