Every once in a while, chain emails or Facebook posts make their way back into prominence. It happened last year with HPV "killing girls." Recently, it seems to be fluoride.

A number of you have been forwarding me a post from last year, which features a meta-analysis published a few years before that on fluoride and IQ.

Fluoride is the topic of this week's Healthcare Triage.

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So let's talk about the Facebook post and the study it's covering. Researchers wanted to look at studies examining a relationship between fluoride exposure and delayed neural behavioral development. They found 27 epidemiological studies that were on point. When they combined the results, they found that people who were supposed to high levels of fluoride had an IQ which was lower by 0.45 points. It was statistically significant.

Now let's discuss. Of the 27 studies, 25 appear to be from China. The other two are from Iran. I bring this up not to disparage those countries' ability to do research, but to point out that they have very different background levels of fluoride then we might see in the United States.

In some of these studies, fluoride levels reached 11.5 mg/L. Compare that to New York City, which shoots for 0.7 to 1.2 mg/L. Moreover, some of the kids got their fluoride from inhaling it from coal burning, or because it was a pollutant.

That's not the same thing as fluoridation here in the United States. These were not randomized controlled trials, but epidemiologic studies. Lots of other factors could be contributing: schooling, arsenic, iodine. Even lead, which often wasn't measured in these studies, could be in play here. We can't say which of these things is "causing" the result.

And what does a difference of 0.45 IQ points really mean? That is really small. In fact, as the researchers point out in the paper, it may be within the measurement error of the test itself.

This isn't the first time people have gotten up in arms about fluoride. Some argue that it has many risks, including those to the brain, thyroid gland, and bones. Others make much more alarmist statements, calling fluoride a "corrosive poison".

Websites and publications raising concerns about fluoride offer "scientific references" and "expert quotes" that really could cause concern that the fluoride in your water not only doesn't help you, but it could also be hurting you.

You know where this is going, right? To the research!

Overwhelming evidence has existed for over 60 years that proves the efficacy of water fluoridation. Some of the best studies come from the early days of fluoridation, because it was easier then to find groups of people who were not exposed to any fluoride sources.

In a 15-year landmark study from Grand Rapids, Michigan, children who received fluoridated water from birth had 50-63% less tooth decay than children from a nearby city in Michigan who drank non-fluoridated water.

Evidence for how well fluoride works is massive. A compilation of the results of 113 studies in 23 countries showed reductions in tooth decay for both baby teeth and adult teeth. People with fluoride in their water had 40–49% less tooth decay in baby teeth and 50–59% less tooth decay in adult teeth.

Another compilation of studies, conducted between 1976 through 1987, showed reduction and tooth decay between 15 and 63%, with the highest benefit for those with baby teeth, but significant benefits for adults who already had their permanent teeth.

The 350 peer-reviewed references compiled by the American Dental Association in their publication Fluoride Facts, also support how well fluoride works.

Of course, preventing cavities might not be worth endangering your help in other ways. So is fluoride really safe?

Once again, the scientific evidence overwhelmingly supports the safety of adding fluoride to a community water supply. Reviews of the safety of fluoride by the Institute of Medicine, the Food and Nutrition Board, the National Research Council of the United States, the US Department of Human Services, Public Health Service, and the World Health Organization, have all led to the conclusion that fluorite supplementation is safe, effective, and recommended for community water supplies.

A systematic review published in 2000 in the BMJ analyze 214 studies on fluoridation and found no evidence of potential adverse effects except for dental fluorosis, which I'll get to in a moment. Periodic reviews every six years by the US Environmental Protection Agency, or the EPA, continue to find no harmful effects related to fluoride in drinking water.

Good scientific studies demonstrate that drinking fluoridated water doesn't increase the risk of hip fractures. More than 50 extremely large studies do not show any association between fluoridation and the risk of cancer.

One small study from the 1950's, which is surprisingly large number of you have read, looked at 15 patients with an overly active thyroid gland, which tried to use large amounts of fluoride as a treatment, and found that it seems to help some patients. On that basis, concerns have been raised about whether fluoride in drinking water adversely affects the thyroid gland.

Again, much better science shows that the answer is no. Studies from people with drinking water with naturally high levels of fluoride found that it had no effect on their thyroid gland size or function, and this matches results from animal studies. Furthermore, two studies found that no association exists between the level of fluoride in water and thyroid cancer.

The National Research Council of the National Academy of Sciences supports the conclusion that drinking optimally fluoridated water is not a genetic hazard. There is no known association between drinking fluoridated water and Down's syndrome.

One psychiatrist in the 1950's published two studies claiming that the two were connected, but four subsequent studies have found no connection, and experienced researchers have noted significant problems with how that psychiatrist analyzed his data.

There's also no generally accepted scientific evidence establishing a link between fluoridated water and other neurological disorders, including attention deficit disorder. There's a study in which rats were administered fluoride at 125 times the level in community fluoridated water, and concluded that the rats showed some behavioral changes. However, this study did not use any sort of controlled group to compare rats who had the fluoride to rats that had not. And scientists who reviewed the results of the study have concluded that it is significantly flawed and cannot be used to draw conclusions about problems with fluoridated water.

There is one real problem that can result from too much fluoride, and that's dental fluorosis. This is a discoloration of the teeth that can occur when a child ingests more fluoride than is recommended. With mild dental fluorosis, the teeth get white flecks or spots, but with severe fluorosis the teeth can get a permanent brown stain.

About 10% of the mild fluorosis seen in children does probably come from the fluoridation of water, although the dentists argue that the small white flecks are a small price to pay for avoiding cavities, tooth decay, missed school and so on.

The bigger cause of fluorosis is likely that kids do sometimes have a habit of swallowing their toothpaste. Fluorosis is the reason that the American Dental Association recommends that children under six do not use more than a pea-sized amount of toothpaste. We assume that little kids are going to follow it, and if they swallow a lot more than that pea-sized amount, they're more likely to get fluorosis. Toothpaste delivers a much more concentrated amount of fluoride than drinking water does, so the biggest cause of fluorosis is likely swallowed toothpaste, not water.

Many experts have concluded the fluoride both beneficial and safe in the drinking water of our communities, and they agree that the risk of fluorosis is far outweighed by the benefits of preventing tooth decay. The American Dental Association, The Centers for Disease Control, the American Medical Association, and the US Surgeon General have all issued statements supporting how well adding fluoride to water works to prevent tooth decay.

In fact, the CDC declared fluoridation of public drinking water to be one of the 10 biggest public health achievements of the 20th century.

Too much of anything can be bad, including fluoride. I don't dispute that. But we shouldn't overlook the fact that dental caries are the most common chronic disease in kids age 6-19. Let's not go backwards. Ignore this chain e-mail.

This episode of Healthcare Triage is supported by audible.com, a leading provider of premium digital spoken audio information and entertainment on the Internet. Audible.com allows users to choose the audio versions of their favorite books, with a library over 150,000 titles. We recommend White Teeth by Zadie Smith. You can download a free audio version of White Teeth, or another of your choice, at audible.com/triage.

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