If it were any other chemical, the news that fluoride may cause cancer would probably have been greeted with little more than mutters about the latest cancer causing substances of the week. But fluoride is no ordinary chemical.
Added to public drinking water for 45 years in an effort to reduce tooth decay, it has been a constant focus of impassioned, often angry, debate, swirling around both political issues of “forced medication” and scientific ones such as fluoridation’s risks and benefits.
Earlier this year, the National Toxicology Program (NTP) released preliminary data from its most thorough and detailed cancer test ever. The results showed that some rats given fluoride in drinking water developed osteosarcomas, rare bone cancers (NEWSWEEK, Feb. 5).
Last week, a panel of outside experts unanimously upheld that conclusion, voting hat there is “equivocal” evidence that fluoride causes cancer. “It’s like there’s a little flashing yellow light on the instrument panel,” said David Rall, director of the National Institute of Environmental Health Sciences, in Research Triangle Park, N.C., which oversaw the study. “It says ‘you better look at this more carefully’.”
“Equivocal” is the term applied when test animals display a marginal increase in cancers that may be related to the chemical.
In the fluoride study, ordered by Congress in 1977, no mice or female rats developed bone tumors. Neither did any of the male rats drinking water containing 0 or 11 parts per million (ppm) fluoride. But one rat on 45 ppm fluoride got osteosarcoma, as did three drinking 70 ppm water. Last week’s panel stamped those results statistically significant. Moreover, unlike other studies that “are often accused of using doses thousands of times greater than [people get],” said Rall, the fluoride levels the rats drank were not extreme. The legal limit for fluoride in drinking water is 4 ppm; it is added to public water supplies at about 1 ppm. More important, the level of fluoride in the animals’ bones — the cancer site — were about equal to that seen in people who have consumed fluoridated water for many years.
Human risk: Now public-health officials will consider other evidence of fluoride’s risks, as well as its benefits. Recent government data show that tooth decay rates are 18 to 25 percent lower in fluoridated than unfluoridated areas, far below the 60 percent or more than proponents often claim. A panel of the U.S. Public Health Service, the principal official promoter of fluoridation, is planning to issue a report on the risks and benefits this July. Its toughest challenge will be assessing evidence of human cancer risk. Even the most sensitive epidemiological study of any substance is lucky to detect a cancer rise of 10 to 15 percent; hardly anyone says fluoride poses that great a threat.
Because of the difficulty of directly observing effects in humans, many of the NTP reviewers emphasized the need to learn all we can from test animals. Lauren Zeise of the California Department of Health Services argued that NTP’s animals could have withstood higher fluoride doses; in a future study, that might more definitively flush out the cancer risk. Ellen Silbergeld of the University of Maryland, noting the lack of cancer in female rats, speculated that females may sequester fluoride in their bones differently from males, keeping it from living bone cells susceptible to carcinogens. Further experiments might shed light on these sex differences.
Until such questions are resolved, the only certainty is that after years in which the government and medical establishment maintained that only quacks questioned fluoridation, the practice is once again a subject for legitimate scientific debate.