Thousands of Americans drink tap water poisoned by unsafe levels of a cancer-causing heavy metal, and government authorities are doing little to stop it, according to a new report from clean water activists.

The chemical hexavalent chromium, also known as chromium-6, gained notoriety as the carcinogenic water contaminant that Erin Brockovich sued a utility over in California—and the new report from advocacy organization Environmental Working Group finds that it shows up in the water systems of major cities all over the country.

The data estimate that water supplies serving 218 million Americans—more than two-thirds of the population—contain more chromium-6 than California scientists have deemed safe. The group estimated that if nothing changes, chromium-6 in tap water will lead to more than 12,000 excess cases of cancer by the end of the century.

The findings are one more sign of a broken state and federal regulatory system that enabled crises in Flint, Michigan, and Hoosick Falls, New York, among other cities where dangerous contaminants in tap water threatened public health, advocates say.

“This is a repeating story of the lack of regulatory oversight and scientific updates to ensure clean drinking water,” Bill Walker, a coauthor of the report, told TakePart. “A basic aspect of public health is being able to drink the tap water. We have not achieved that in the U.S.”

The EWG report analyzes data on chromium-6 collected for the first time under a federal monitoring rule passed four years ago. The current federal standard for chromium in drinking water was set in 1991, when the U.S. Environmental Protection Agency set a limit of 100 parts per billion for total chromium, including chromium-3, a less threatening contaminant. Back then, chromium was linked to skin rashes, but the potential cancer threat posed by the pollutant had not been thoroughly researched. Knowing what we know now, activists say the federal standard is meaningless.

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Despite mounting evidence that chromium-6 is carcinogenic, only one state has passed a limit focused on this contaminant, which can occur naturally or seep into the water supply through industrial pollution. California began enforcing a chromium-6 limit of 10 parts per billion two years ago.

Citing instances of cancer helped Brockovich build a case against Pacific Gas and Electric Company over contaminated water in Hinkley, California, in 1993. For her, the looser federal standards on chromium-6 are disappointing.

“This is an abject failure by the EPA, including members of Congress charged with overseeing the agency, and every American should be outraged by this inaction,” Brockovich said in a statement released by EWG.

California scientists also came up with a considerably more stringent but unenforceable goal for the contaminant: Set at 0.02 parts per billion, it was designed to prevent more than one in 1 million people from getting cancer if the person consumed a half gallon of water daily over 70 years. The goal was established as part of a health proceeding before official rules were set, a common precursor to regulatory action. The final rulemaking proceeding considers not just health risks but also what is achievable for industry—and industry balked at meeting the standard, resulting in the looser standard and the lingering frustrations of activists.

The EPA says its work on chromium-6 is ongoing. It is studying the potential risks linked to the contaminant, which may be a step toward regulating the chemical. The agency expects to release a draft of its risk report for public comment next year, according to Monica Lee, EPA press secretary, in a statement emailed to TakePart.

She also pointed out that data gathered by the EPA show most U.S. drinking water systems are beating California’s enforceable standard for chromium-6—the 10 parts per billion standard. “While this standard only applies to water systems in California, less than 2 percent of the systems that collected data under [the 2012 monitoring rule] reported hexavalent chromium at levels exceeding this standard,” she wrote.

In many cities, chromium-6 sample data exceeded the public health goal set by California scientists but not the enforceable limit, according to the EWG report, which ranked water systems serving more than 1 million customers. The worst offenders: City of Phoenix, Missouri American Water (St. Louis County), City of Houston, City of Los Angeles Department of Water and Power, and Suffolk County Water Authority in New York.

Yet in Phoenix and St. Louis, water officials say customers should not worry.

The average level of chromium-6 in Phoenix water samples was 7.853 parts per billion, well above the public health goal set by California scientists. The authors of the EWG report said they did not study why the levels are higher in some cities than others and that it would be hard to extrapolate where the pollution is coming from based on the data they used.

Susan Kinkade, a civil engineer in the environmental services division at the City of Phoenix Water Services Department, cast doubt on the usefulness of the figures in the EWG report, emphasizing that Phoenix’s water is safe to drink and that the system is abiding by federal rules.

“That number is skewed,” she said in a phone interview with TakePart. “That is an average of all the samples we collected. We sampled at groundwater wells, surface water points, and within the distribution system. The highest number came out of our groundwater wells, but that’s only 2 percent of our supply.”

Kinkade said there are natural reasons chromium-6 shows up in Phoenix samples. “It’s a naturally occurring element. The Southwest is one of the regions in the U.S. with the highest level of naturally occurring chromium. The wells are situated where there is a natural chromium deposit. It’s the geology. It’s not industrial contamination,” she said.

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Missouri American Water had the second-highest average chromium-6 levels for water systems serving more than 1 million customers. The average was 1.258 parts per billion. But Brian Russell, a spokesperson for Missouri American Water, emphasized that it is safe to drink the water in St. Louis.

“We test for total chromium, which includes chromium-6, every year as required and consistently come in well below EPA requirements for safe drinking water, and we tested specifically for chromium-6 only in 2013 and found levels that ranged from 0.9 to 1.6 parts per billion on average. There are no federal requirements to test for chromium-6, or prescribed levels to achieve, but at that level we even come in below the state of California’s stringent level of 10 parts per billion,” he wrote in an email to TakePart.

Russell added that it is unclear how chromium makes its way into the water supply in his system. “It occurs naturally from the breakdown of a number of different products and via industrial processes, so trying to name the exact source would be little more than hypothesis on our part,” he said.

Nevertheless, the EWG report authors have clear-cut advice for residents of Phoenix and St. Louis County: Use water filters.

“In areas like St. Louis or Phoenix, people should not wait around for a cleanup by their local agencies. They should take action to protect themselves by buying a home water filter,” said Walker, the report coauthor. He uses water filters in his home and recommended the watchdog group’s buying guide for the products.

Kinkade, the Phoenix official, warned about the potential downsides of filters.

“I always caution customers about point-of-use water filters if the customers aren’t good at maintaining them,” she said. Failing to follow the directions can create a “little factory for microbes growing under your kitchen sink.”

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For the EWG report authors, the data highlight a problem that is bigger than chromium-6. They see a regulatory system hijacked by industry influence at the state and federal levels.

“The process of setting standards has been sandbagged by the chemical and electrical power industries,” Walker said.

“It’s extremely surprising and frustrating that as the science changes, including what we know about the toxicity and its prevalence of this contaminant, there’s the inability of our government to keep pace with that change,” report coauthor David Andrews said.

Bryan Goodman, a spokesperson for the industry group American Chemistry Council, told TakePart by email that the data available on the health effects of chromium-6 are limited.

“Because of the limited scientific data available to inform how low environmental levels of this naturally occurring substance could impact human health, ACC supported a third party research organization to undertake a large, multiple institution research study. ACC believes [regulators] should consider the entire scientific database on hexavalent chromium as they make regulatory decisions about this substance,” he wrote.

In setting their public health goal, scientists in California cited studies that link chromium-6 to cancer. For instance, they said their analysis of data from China “found increased rates of stomach cancer in people exposed to high levels of chromium-6 from drinking water.”