Last week 5.2 million Americans learned that their drinking water is contaminated with man-made chemicals linked to cancer. The Environmental Protection Agency issued a health advisory for two compounds: perfluorooctanoic acid (PFOA), which is used in the manufacture of Teflon and other nonstick substances, and the related perfluorooctane sulfonic acid (PFOS). An E.P.A. health advisory is not a regulation; it is nonbinding and nonenforceable. It does, however, require a public water system to notify its customers of the presence of the chemical and the dangers it poses. As a result, the E.P.A.’s announcement had immediate effects. Within hours, public wells were shut down in Horsham, Pa., and Maricopa County, Ariz. West Virginia’s Bureau for Public Health ordered a “do not drink” advisory for the water in three communities: Parkersburg, the site of a Teflon factory that until recently was operated by DuPont; the adjacent town of Vienna; and Martinsburg, four hours east, near the Maryland border. The West Virginia National Guard sent convoys of tankers containing drinking water to Vienna.

Rob Bilott, a lawyer at the Cincinnati firm of Taft, Stettinius and Hollister, has demanded that the E.P.A. take action on PFOA since 2001. (See “The Lawyer Who Became DuPont’s Worst Nightmare.”) By that point, Bilott had read more than 110,000 pages of internal corporate documents related to PFOA. He had learned that DuPont, despite knowledge that the chemical was linked to increased rates of cancer and other horrific health conditions in animals and human beings, had dumped mountains of the stuff into the local water supply for decades. It was even known in 2001 that perfluorinated chemicals had been detected nationwide in nearly every sample tested at blood banks. At least once a year since then, Bilott has written to the E.P.A. to renew his request.

Anyone who wants to understand how seriously the United States government considers its duty to protect the health of its citizens need only review the history of the correspondence between Bilott and the agency. It is a one-sided correspondence. In his annual letters, Bilott attaches previous correspondence, new scientific findings about the dangers of PFOA and disclosures from the legal cases he has waged continuously against DuPont since 1998. The E.P.A.’s responses are curt, usually less than a single page. In recent years the E.P.A. has claimed to be close to issuing a health advisory. But it did not do so until last Thursday.

Who has been drinking this poisoned water? More than five million American citizens, according to the Environmental Working Group, which analyzes data released publicly by the E.P.A. It has been found in dangerous concentrations in 52 public water systems across 19 states and two Pacific island territories. But Bilott said that the new E.P.A. advisory — warning against long-term exposure to drinking water with a concentration of PFOA or PFOS higher than 0.07 parts per billion — is too conservative. “It needs to be much, much lower,” he told me this week. “Even at the lowest detectable levels, it still builds up in the blood over time.” The Environmental Working Group has proposed a limit of 0.001 parts per billion, though Bilott suspects that anything above zero may be dangerous, because of PFOA’s extraordinary “bio-persistence” in the blood. Once it enters your body, it stays there.