Self-styled environmental and consumer advocates often are long on ideology but short on science. The latest example is a study by the notedly alarmist Environmental Working Group, which found a chemical called hexavalent chromium (also known as Chromium-6) in the tap water of 31 out of 35 cities sampled nationwide.

This kind of study – in which one looks for a certain chemical in the air, water, or humans' bodies – is increasingly common, but for various reasons, its significance is questionable. First, analytical techniques have become so sensitive that they can detect amazingly minuscule amounts of almost anything. Second, even if it's found in our bodies, the mere presence of a substance does not mean it's harmful. And that applies even to chemicals that can be deadly at high levels.

Consider botulinum toxin, for example, which in food can cause botulism, a particularly lethal form of food poisoning. In tiny amounts, however, it is a valuable pharmaceutical, used to treat muscle spasms and to remove wrinkles, known by its brand name of Botox.

Another example is radioisotopes. At high levels, they can generate dangerous radiation, but there are small amounts of radioisotopes – naturally-occurring variants of elements such as hydrogen and carbon – in every cell of our bodies. And there is a radioactive form of potassium in all glass. But that shouldn't alarm us; nor, in any case, can we do anything about it. We live in a sea of chemicals; indeed, chemicals are us!

Chromium-6 can be harmful if inhaled, and probably if ingested in large amounts. Water with high concentrations of Cr-6 fed to rodents causes gastrointestinal tumours, indicating that it is a carcinogen – in those species. But rodents are not little humans with tails. Toxicological findings often fail to translate well from one rodent to another, let alone from rodents to humans.

It is useful to consider what Californian regulators have to say about Cr-6. They have proposed a "Public Health Goal" (PHG) for Cr-6 of 0.06 parts per billion (ppb) in water, which is the estimated "one in one million" lifetime cancer risk level. That means that "for every million people who drink two litres of water with that level of Cr-6 daily for 70 years, no more than one person would be expected to develop cancer from exposure to Cr-6". They picked this very conservative, very low level because "the 'one in one million' risk level is widely accepted by doctors and scientists as the 'negligible risk' standard".

But you would never know that from the hyperbolic pronouncements of the Environmental Working Group, which inaccurately called the proposed California PHG for Chromium-6 a "safe maximum recently proposed by California regulators" and decried the fact that water in 25 of the 35 cities tested was found to contain higher concentrations. For their own purposes – that is to say, alarmism and fear-mongering (and, possibly, fund-raising) – they completely ignored California regulators' blunt admonition:

"A PHG is not a regulatory standard. It is only one step in the process of developing an enforceable standard that is set by the California department of public health for drinking water that public water systems must meet."

But let's put aside, for a moment, the hand-waving about the distinction between goals and standards, and rodents and humans. We have the advantage of data from what amounts to a long-term, real-world experiment in which humans were exposed over a long period to Cr-6 in groundwater. This was in Hinkley, California, where the Pacific Gas and Electric Company was accused of leaking Chromium-6 into the town's groundwater for more than three decades. Eventually, the company paid $333m in damages to more than 600 townspeople and pledged to clean up the contamination. The litigation made a celebrity of activist and para-legal Erin Brockovich, whose story was made into a movie that starred Julia Roberts.

But here's the denouement that both the litigation and the film missed: a California Cancer Registry survey released this month failed to find a disproportionately high number of cancers in Hinkley. On the contrary, from 1996 to 2008, 196 cancers were identified among residents of the census tract that includes Hinkley — more than 10% fewer than the 224 cancers that would have been expected given its demographic characteristics. Such surveys are probably not highly accurate, but this one does tell us that if Cr-6 in water is a human carcinogen, it is certainly not a potent one.

So much for the EWG's latest earth-shaking, the-sky-is-falling exposé.