Another article mentions a number of studies:
A June article in the Proceedings of the Pennsylvania Academy of Science by attorney John R. Graham, Burk, and Pierre J. Morin, former scientific adviser for the minister of environment in Quebec, also reviews this controversy. It concludes that, compared with the unfluoridated cities, there is an excess of 20 to 30 cancer deaths per 100,000 people who live in the major fluoridated cities of the U.S. for at least 15 to 20 years.
Several investigators have looked for a more specific relationship between stomach cancer and fluoridation. The hypothesis is that fluoride would be more likely to cause stomach cancer than any other type because fluoride in the stomach forms hydrofluoric acid, a powerful irritant that is mutagenic in several in-vitro lab tests. In 1978. CDC’s Erickson, after correcting for age, race, and sex, found the death rate from cancer of the digestive system was 9% higher in cities with fluoridated water. However, when he subtracted all subjects with Asian and Hispanic surnames and corrected for education and population density, the excess disappeared.
The Knox report, a comprehensive review of most fluoride cancer studies that was completed in 1985 by the Royal College of Physicians in England. concludes that there is no convincing evidence that cancer death rates are higher in areas with fluoridated water. Thus, as with most environmental agents that have been studied for their effects on cancer, the results for fluoride are still inconclusive.
Now, this article shows up on the fluoridealert.org website, which can be reasonably characterized as an anti-fluoridation website. In light of this, what does "inconclusive" mean? Graham, Burk, and Morin find fluoridation responsible for "an excess of 20 to 30 cancer deaths per 100,000" due to fluoridation. Critics believe this excess is accounted for by other factors which were not properly figured in.
Interestingly enough, the National Cancer Institute lists the U.S. death rate from stomach cancer at about 4.5 per 100,000. This implies a death rate in fluoridated cities that is between 5 and 8 times higher than it is in nonfluoridated cities. Surely, someone would have noticed that by now.
Also, since about 2/3 of the United States population receives fluoridated water, let's run some numbers.
Take the minimum of 20 "excess" deaths due to stomach cancer. Multiply by two-thirds to get 13.3. This is close to three times what we actually observe.
This claim keeps getting stuck in my garbage filters.
On the other hand, we have a report which says:
...Several of these correlational studies have evaluated cancer mortality data from cities in the U.S. with differing concentrations of fluoride in their water supplies. In 1975, Yiamouyannis and Burk78 initially reported that cancer mortality was higher in ten U.S. cities with fluoridated water supplies than in 10 cities without fluoridated water supplies. The National Cancer Institute (NCI) reanalyzed these cancer mortality data using different statistical methods and found no significant increases in cancer mortality in the cities with fluoridated water.124 In addition, Hoover and co-workers evaluated cancer mortality in Texas, comparing counties with higher concentrations of naturally occurring fluoride in their water supplies (0.7 to 1.2 mg/1; 1.3 to 1.9 mg/1; >2.0 mg/1) with counties with lower concentrations (<0.7 mg/1) and found no evidence of increased cancer mortality in the counties with higher concentrations. The upper limit of fluoride concentration was not defined in this study.
Due to the controversy over these conflicting results, the cancer mortality data for U.S. cities have subsequently been reanalyzed by several other researchers utilizing slightly different techniques and/or modifications in the area or years being compared. These subsequent reports have supported the NCI conclusions in finding no significant increase in cancer mortality in the cities with higher concentrations of fluoride.125-132 Correlational studies evaluating populations in other countries also support these findings.
Another study evaluated the cancer mortality in Anglesey, where the water supply was fluoridated (0.9 mg/1) over the period 1955 to 1966, comparing cancer mortality in the period before fluoridation (1949 to 1953) with that in a period more than 20 years after fluoridation (1979 to 1983).137 This study showed no evidence of increased mortality from respiratory cancer, cancer of the stomach, or cancer of all other sites in Anglesey when compared with cancer mortality in England and Wales for the same period. Another British study evaluated cancer mortality (all cancers combined plus 12 specific cancer sites) from 1969 to 1973 for 67 small areas in England, based on the concentrations of naturally occurring fluoride in their water supplies (categories: ≤ 0.2 vs. ≥ 1.0 mg/1; ≤ 0.1 vs. 0.5 to 0.99 mg/1) and found no evidence of increased cancer mortality with increasing concentrations of fluoride.138
Now, one of the problems here is that a publishable result is one where the observed result has a significance level below 0.05. That means there is a 5% chance (one time in twenty) that the observed result is due to chance alone. This means if you do twenty studies on random noise, on average one of them will show a result that is significant below 0.05. Now, one person may not set out to do twenty studies in a row with the intention of publishing only those that show some result, but it's easy enough to do by accident. Suppose, for example, you compare a fluoridated city with a non-fluoridated city. You then pore over a list of a thousand different diseases. If there is no correlation, you will find that, on average, fifty diseases show a significant relationship, and half of those will be in either direction. Thus, if you want to prove fluoridation is good, 25 of the 1000 diseases will occur at a significantly lower rate in the fluoridated cities. And if you want to prove it's bad, the same list will give you 25 diseases that occur at a significantly higher rate.
And it's all random noise.
A big problem here is due to the success of epidemiology. All the big effects have been found, and the only thing left is the discovery of tiny effects that are often buried in the noise, and it's been estimated that up to half the published results in the journals are only noise.
Now, an interesting note: I mentioned another research finding, which has also received a fair amount of attention: Women who have had abortions are 50% more likely to develop breast cancer.
On hearing that, glittercat13 asked, "What right-wing group came up with that one?"
Answer: I don't know. I'm pretty sure it wasn't a "right-wing group" that came up with the association, but a number of anti-abortion groups have been perfectly happy to repeat it.
But that's actually beside the point.
The point I find the most interesting is, the study showing a relationship between abortion and increased breast cancer risk is denounced as a "right wing group's finding". On the other hand, despite the prevelance of highly vocal anti-fluoride groups, the study showing an increase in stomach cancer rates is gospel.