The Environmental Working Group welcomes informed scrutiny of our recent study on hexavalent chromium (chromium-6) in tap water. We detected this probable human carcinogen in the water of 31 of 35 American cities tested.

We now know that chromium-6 exposure is not limited to communities like Hinkley and Kettleman City, California, victims of extreme industrial pollution and corporate malfeasance. Our results show that communities across the United States, and probably around the world, may be exposed to low levels of this toxin. Worldwide, known hotspots of chromium-6 contamination include Glasgow, Oinofita, Greece and parts of India, China and Australia.

The US Environmental Protection Agency reacted swiftly to our study with a four-point plan to help water utilities nationwide monitor and assess chromium-6 levels, and EPA chief Lisa Jackson has pledged to move quickly to set a nationwide safety standard.

California officials recently lowered the proposed safe level from 0.06 parts per billion (ppb) to 0.02 ppb. The amount EWG found in tap water from one American city, Norman, Oklahoma, was nearly 650 times higher than this. State scientists concluded that the lower goal is necessary to account for the special sensitivity of infants and young children to carcinogens. EWG and many other scientists and public health advocates have urged exactly this approach. Establishing this public health goal is the first step in setting a mandatory safety standard, which, under California law, should have been done by 2004.

The author of a recent opinion piece here argued that the proposed safe level would achieve an insignificant reduction in lifetime cancer risk. Public health agencies disagree. Safety goals are intended to protect people over a lifetime of exposure, not just to chromium-6 but to the many other cancer-causing compounds that commonly contaminate tap water, including chlorination byproducts linked to bladder cancer, and arsenic linked to skin, bladder and lung cancer. Many of these compounds also contaminate food, air and soil, or turn up in consumer products. Over a lifetime, people's exposures to all these sources add up.

Stringent safety standards aren't a luxury. Forty-one percent of all Americans will be diagnosed with cancer during their lifetimes. About 21% will die from it, according to the US National Cancer Institute; in 2009 alone, 1.5 million people were diagnosed. Health officials can hardly be too protective when it comes to tap water, which is widely consumed and commonly contaminated. We don't know how many cancer cases are linked to chemical exposures, but in an April 2010 study the President's Cancer Panel found that environmental causes of cancer are "grossly underestimated" and "needlessly devastate American lives".

The case of chromium-6 is particularly troublesome. Both animal and human studies have shown it to be a potent carcinogen. As far back as 1987, researchers documented an increased risk of stomach cancer and a "significant excess of overall cancer mortality" among Chinese villagers whose water had been polluted by chromium-6. In 2008, a gold-standard study (pdf) by federal scientists found increases in gastrointestinal tumors in rats and mice exposed through drinking water. Based on that data, the US National Toxicology Programme found that chromium-6 shows clear evidence of carcinogenic activity.

Certainly, the actions necessary to address the problem will carry significant costs. But the accumulating evidence makes clear that simply ignoring it is not an option. The first step is to identify those water supplies that contain unsafe levels of chromium-6, and the EPA deserves credit for following up promptly on our findings. The second step is to find ways to minimise contamination where it is found.

But providing safe drinking water is not just a matter of treatment or purification. As a nation, we need to protect our water supplies at the source. We spend 1,900 times more to treat drinking water than we do to protect it in the first place. Our priorities are back to front.