The study, conducted by the National Toxicology Program and overseen by the American Public Health Service, was designed to determine whether water fluoridation holds an increased cancer risk in humans. Rats and mice were chosen as the study subjects. Particular attention was paid to mouth, liver and bone cancers.The results, interpreted by John Yiamouyiannis, president of the Safe Water Foundation in Ohio, writing in the Townsend Letter for Doctors, an American journal, showed among the rats exposed to the fluoridated water:

Precancerous changes in oral squamous cells.

An increase in the incidence of tumours and cancers in the mouth.

A rare form of bone cancer.

An increase in the tumours in the thyroid.

The mice exposed to fluoridated water had a rare form of liver cancer.

"The types of cancer caused by fluoride in rats and mice may be entirely different than the types of cancer caused by that same substance in humans," wrote Yiamouyiannis, who has performed epidemiological studies on the effects of fluoride.

The animals were given higher doses of fluoride than humans would be exposed to in order to adjust for the short time of their exposure to fluoride and because, according to the study, says Yiamouyiannis, "man is generally more vulnerable" than the experimental animals to the carcinogenic effects. Nevertheless, the doses of fluoride linked to a higher cancer risk were between l/l0 to l/50 of that of benzene, another, established carcinogen.