There’s a controversy going around whether the benefits of water fluoridation outweigh the potential harms.
What is the evidence for the harm this may cause and what evidence supports the health benefits?
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Typically No, but it depends on the dose.
The World Health Organisation released a fairly comprehensive report (PDF Warning) called "Fluoride in Drinking-water" that covers this topic.
I apologise for quoting so extensively from this report, but, well, it's excellent. I'd suggest anyone that is skeptical, have a look at the report and the (many) papers it cites to justify its findings.
Firstly, fluoride is naturally occuring in all types of water.
Seawater typically contains about 1 mg/l while rivers and lakes generally exhibit concentrations of less than 0.5 mg/l. In groundwaters, however, low or high concentrations of fluoride can occur, depending on the nature of the rocks and the occurrence of fluoride-bearing minerals.
The major risk cited by fluoride-in-water-skeptics is the risk of fluorosis. Unfortunately for them, this only occurs after excessive ingestion of fluoride. This typically occurs in regions that do not control the levels of fluoride in their drinking water and the levels are naturally high. Typically the effects are worse in hotter regions where people require more water.
Fluoride has beneficial effects on teeth at low concentrations in drinking-water, but excessive exposure to fluoride in drinking-water, or in combination with exposure to fluoride from other sources, can give rise to a number of adverse effects. These range from mild dental fluorosis to crippling skeletal fluorosis as the level and period of exposure increases. Crippling skeletal fluorosis is a significant cause of morbidity in a number of regions of the world.
The beneficial and the detrimental effects of fluoride naturally present in water were well established by the early 1940s. High levels of fluoride present in concentrations up to 10 mg/l were associated with dental fluorosis (yellowish or brownish striations or mottling of the enamel) while low levels of fluoride, less than 0.1 mg/l, were associated with high levels of dental decay (Edmunds and Smedley, 1996), although poor nutritional status is also an important contributory factor.
What most of the governments in fluoridated-water countries do is attempt to find a middle ground between too much fluoride (causing fluorosis):
in China some 38 million people are reported to suffer from dental fluorosis and 1.7 million from the more severe skeletal fluorosis (WRI, 1990). In India, Susheela and Das (1988) suggested that around one million people suffer from serious and incapacitating skeletal fluorosis. Using the Chinese dental:skeletal fluorosis ratio, India could therefore have up to 20 million dental fluorosis sufferers.
and too little (contributing to high levels of dental decay):
The level of dental caries (measured as the mean number of Decayed, Missing or Filled teeth) falls from seven at a fluoride concentration of 0.1 mg/l to around 3.5 at a fluoride concentration of 1.0 mg/l. As fluoride concentration increased further (up to 2.6 mg/l) dental decay continues to fall, but only slightly (Dean, 1942; USPHS, 1991).
For a government, based purely on cost I would expect it's worth controlling the fluoride levels in drinking water. The cost of treating the water is lower than treating severe fluorosis or unnecessary dental caries, particularly if millions of people are likely to be affected.
The WHO therefore recommends that a moderate level of fluoride be maintained in drinking water:
In 1984, WHO conducted an extensive review and found that there were insufficient data to conclude that fluoride produces cancer or birth defects. In addition, WHO noted that mottling of teeth (i.e. dental fluorosis) is sometimes associated with fluoride levels in drinking-water above 1.5 mg/l and crippling skeletal fluorosis can ensue when fluoride levels exceed 10 mg/l. A guideline value of 1.5 mg/l was therefore recommended by WHO as a level at which dental fluorosis should be minimal (WHO, 1984).
The 1.5 mg/l fluoride guideline value that was set in 1984 was subsequently re-evaluated by WHO and it was concluded that there was no evidence to suggest that it should be revized (WHO, 1996, 2004). The 1.5 mg/l guideline value of WHO is not a “fixed” value but is intended to be adapted to take account of local conditions (e.g. diet, water consumption, etc.).
The same report actually covers the (pretty ridiculous) claims of birth defects, cancer, etc. and debunks them. As they're not specifically claimed in the question, I'll leave it to people to read the report if they're interested.
I would suggest people also read @Oliver_C's post on a similar question for more information.
Skeptic would undoubtedly want to know about the systematic reviews done on fluoridation. They are listed here on the CDC website:
In short, these reviews find that fluoridation is cost-effective and safe. There are some recommendations for further research.
Item 1 Check out the 2006 National Research Council publication, Fluoride in Drinking Water: A Scientific Review of EPA Standards. Here is the brief: http://dels.nas.edu/resources/static-assets/materials-based-on-reports/reports-in-brief/fluoride_brief_final.pdf
Item 2 The vast majority of scientists were anti-fluoride in the beginning. Over the years, many leading pro-fluoridationists changed their opinions based on science.
Dr. John Colquhoun, Chief Dental Officer of Auckland was one. In this 1998 interview, he gives first hand account of how the data was manipulated to fit preconceived notions. Especially interesting is how the Hastings study changed methods and then threw out the control group because it didn't support the result they wanted: http://fluoridealert.org/fan-tv/colquhoun/
Item 3 People need to pay attention to KIDNEYS as well as cavities. Renal Failure and Fluoride in the Oxford Journal of Nephrology Dialysis Transplantation: http://ndt.oxfordjournals.org/content/23/1/411.1.full