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, including in a sample from San Jose.

We now know that chromium-6 exposure is not limited to communities like Hinkley and Kettleman City, victims of extreme industrial pollution and corporate malfeasance. Our results show that communities across the United States may be exposed to low levels of this toxin.

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

California just lowered its proposed safe level from 0.06 parts per billion to 0.02 ppb — the amount we found in San Jose tap water is nearly 70 times higher. State scientists concluded that the lower goal is necessary to account for the special sensitivity of infants and young children to carcinogens. Environmental Working Group 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.

Jeff Stier and Henry I. Miller wrote an op-ed for this newspaper (Opinion, Jan. 11) that 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 Americans 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 percent will die from it, according to the National Cancer Institute. In 2009 alone, 1.5 million were diagnosed. Health officials can hardly be too protective when it comes to tap water, which is widely consumed and commonly contaminated.

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 by federal scientists found increases in gastrointestinal tumors in rats and mice exposed through drinking water. Based on that data, the National Toxicology Program found that chromium-6 shows clear evidence of carcinogenic activity.

Clearly, 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 minimize 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 to protect it in the first place. Our priorities are backward.

REBECCA SUTTON is a senior scientist with the Environmental Working Group. She wrote this article for this newspaper.