CHICAGO - Cancer-causing chromium is turning up in tap water in more than two dozen cities, according to a study that urges federal regulators to adopt tougher standards.

Even though scientists at the U.S. Environmental Protection Agency and National Toxicology Program have linked the ingestion of hexavalent chromium to cancer, the EPA doesn't require cities to test for the toxic metal.

The Environmental Working Group hired an independent laboratory that found the metal in treated drinking water from 31 cities.

The amount in Lake Michigan-Chicago water pumped to 7 million people was 0.18 parts per billion, three times higher than a limit California officials proposed last year.

A handful of other cities were significantly above the proposed California limit, including Norman, Okla.; Honolulu; Riverside, Calif.; and Madison, Wis., according to a report to be released today.

The new findings could pose another challenge for utilities that are detecting dozens of unregulated substances in treated drinking water, including pharmaceutical drugs and industrial chemicals that can pass unfiltered through conventional treatment methods. Chromium can be found naturally in the environment but also is released by industry into waterways.

While the potential health threats of many pollutants are still being studied, researchers say there is a clear risk of stomach cancer from drinking water contaminated with hexavalent chromium