In their need to understand the risks of chromium-6, Hinkley residents must navigate the complex world of science, with full knowledge that there's a long history of scientists revising their opinions of what's safe and what's not.

For example, in 1948, a Swiss scientist was awarded the Noble Prize in physiology and medicine for recognizing the insect-killing properties of DDT.

Twenty-four years later, the miracle chemical in the battle against insect pests was banned for agricultural use in the United States after the realization that it caused significant damage to many animal species, including birds and amphibians, and perhaps human health as well.

Remember thalidomide? Many Hinkley residents do.

The sleeping pill introduced in the early 1950s was found to minimize the effects of morning sickness in many pregnant women. Sales of the drug boomed in many countries in the late 1950s until 1962, when it was abruptly pulled from the market.

This former friend to expectant mothers was found to cause many forms of birth defects.

The story of DDT and thalidomide, and other drugs like them, led to tighter controls of medicine before releasing them to the public, said Heriberto Robles, an Irvine-based board certified toxicologist with 32 years experience in environmental toxicology and human health.

DDT and other chemical disasters led then-president Richard Nixon to create the U.S. Environmental Protection Agency in 1970, said Robles, who has spoken to the Hinkley community about the effects of chromium-6 on human health.

Following a presentation late last year at the Hinkley Elementary/Middle School, Robles was asked if he would drink the water, straight from the tap, in Hinkley.

Yes, he would, he told the questioner, but his wife probably wouldn't, Robles recalled.

"The difference is knowledge," said Robles, who has a Ph.D. in toxicology and who founded Enviro-Tox Services Inc. in 2000.

"Regulatory agencies err on the side of safety," said Robles, who added that he would feel comfortable drinking water out of the tap as long as it did not exceed the current state standard, which is 50 parts per billion total chrome.

Nevertheless, he said, "the lower the better."

The human body's stomach is capable of changing chromium-6 into chromium-3, so long as the number is in line with the state standard, he said.

He would not feel as comfortable drinking water that was at the federal limit of 100 parts per billion.

"Chromium-6 can contribute to cancer anywhere in the body, depending on genetic susceptibility and level of exposure," said Max Costa, chairman of the department of environmental medicine at New York University and program director and professor of the NYU Cancer Institute.

Inhaled chromium-6 attacks the nose, hence the prevalence of severe nosebleeds in people with exposure to it, he said.Showering or swimming in chromium-6 contaminated water can lead to inhalation of the toxic chemical, Costa said in a telephone interview.

Although Hinkley residents assert chromium-6 is the cause of a variety of illnesses in themselves and relatives, there's no solid study to confirm that belief.

A 2010 toxicological review of chromium-6, developed for the federal EPA, cited numerous flaws in several large scale-studies of people who were exposed to significant amounts in drinking water.

Studies of chromium exposure at Hinkley, Topock and Kettleman City were invalid, for example, because researchers used ZIP code populations and didn't isolate people exposed to the pollution, the report said.

No current residents' well is close to the state of California's maximum contaminate level for total chromium, which is 50 parts per billion, half that of the federal EPA's, which is 100 parts per million, said Raudel Sanchez, a doctorate-level chemical engineer, employed by Brea-based Project Navigator Ltd., an international environmental engineering consulting firm. The firm has been hired by Pacific Gas & Electric Co. to advise the Hinkley community on scientific issues.

"I can't think of any resident whose well registers 10 parts per billion," Sanchez said. Most are significantly below that amount, he said.

People who live within a mile of the plume's most recent boundary and have even a trace amount of chromium-6 in their well can request bottled water from PG&E, which will be delivered to their home for free.

Sometime this month, the California Department of Public Health is expected to announce a proposed maximum contaminate level for only chromium-6, not total chromium.