The Environmental Protection Agency has asked local U.S. communities to test more carefully for hexavalent chromium, a probable carcinogen.
After preliminary health studies, the EPA opted Wednesday to classify the chemical known as chromium-6 as one likely to cause cancer in humans when ingested over the course of a lifetime.
It adopted a rule of a maximum 100 parts per billion, and urged managers of water systems with their source in ground water be tested two times a year, versus four times a year for systems with surface water sources.
"EPA's latest data show that no public water systems are in violation of the standard," the agency said in a statement.
Still, a private US environmental group has found that drinking water in many American cities contains hexavalent chromium, The Washington Post reported last month.
The study by the Environmental Working Group — the first nationwide analysis measuring the presence of the chemical in US water systems — found hexavalent chromium in the tap water of 31 out of 35 cities sampled.
Of those, 25 had levels that exceeded the goal proposed in California, which has been aggressively trying to reduce the chemical in its water supply.
Hexavalent chromium has long been known to cause lung cancer when inhaled, and scientists recently found evidence that it causes cancer in laboratory animals when ingested. It has been linked to liver and kidney damage in animals, as well as leukemia, stomach cancer and other cancers.
A widely used industrial chemical until the early 1990s, hexavalent chromium is still used in some industries, including chrome plating and the manufacturing of plastics and dyes. The chemical can also leach into groundwater from natural ores.
The chemical compound was first made famous in the hit 2000 Hollywood movie "Erin Brockovich" about the eponymous environmental crusader.