Supplemental Fluoride: Benefits and Harms
Evidence for a population level benefit of community water fluoridation is extensive and has been well-reviewed elsewhere.The overarching conclusion from this evidence is that fluoridation is effective in reducing the prevalence and severity of dental caries across the lifespan, even after the expanded exposure to fluoride from toothpaste and other nondietary sources.
However, closer inspection of subpopulations of minority children demonstrates that although the DMFT index has declined in the general population, it has remained stubbornly high in some areas. Subgroups of children in the top tertile for the DMFT index include those who are:
Black or Hispanic;
Living in poverty;
Lacking access to dental care; and
Living in areas with a nonfluoridated water supply.
Lack of fluoridation has been found to have the greatest influence on high DMFT index, but as children get older, race, poverty, and lack of dental insurance outstrip the effects of nonfluoridation. In other words, although drinking water fluoridation is beneficial, it cannot completely overcome the deleterious consequences of social inequality.
Claims of Harm
Despite substantial evidence for benefit, healthcare providers often find it challenging to convince parents that no harm will come to them or their children from recommended amounts of supplemental fluoride. These parents may be aware of the views of contemporary "antifluoridationists" who believe that water fluoridation is a vast conspiracy among scientists, health professionals, and industry (such as aluminum mining and pharmaceuticals) to dump an unnecessary and dangerous waste product into public water supplies. Fluoride has been blamed for allergies; goiter; chronic fatigue syndrome; hip fractures; Down syndrome; and many types of cancer, most notably osteosarcoma.
Fluoride's detractors insist that fluoride has no public health value and that no clear scientific evidence proves that fluoride actually reduces tooth decay or is harmless to humans. Some have even claimed (incorrectly) that the American Dental Association can arbitrarily revoke the licenses of dentists who voice disapproval of fluoridation.
Objections to community fluoridation are based on poorly designed studies whose conclusions are quoted repeatedly and without objective analysis. Critics who cite these studies often do not distinguish correlation from causation. The most often cited harms from fluoride involve intelligence, bone health, reproductive health, and cancer.
Fluoride and intelligence. A study widely cited for fluoride's deleterious effects purports to show that fluoride impairs intelligence in children. The study compared 118 children in 2 Chinese villages: one with high and one with low naturally occurring fluoride levels in the water supply. Group comparisons were made on 7 categories of the intelligence quotient (IQ) in these 2 villages. The investigators found that IQ was significantly lower in the high-fluoride village and that IQ was inversely related to urinary fluoride levels. However, this study has major limitations. Without elaborating on the criteria for inclusion or exclusion, the researchers stated that they used a "cluster" sampling method to choose the children from each village. This technique resulted in fewer than 5 children in some cells, eliminating the possibility of determining significance. The fluoride level in the drinking water of the high-fluoride village was up to 4 times the amount recommended by US standards. Other environmental sources of fluoride exposure, including the use of coal with high fluoride content in poorly ventilated home stoves, was not taken into account. Furthermore, the village with high fluoride levels also had low iodine levels, a potentially confounding variable. Insufficient maternal, fetal, and childhood iodine intake is associated with low intelligence.
Fluoride and reproductive health. Much of the research on the biological effects of fluoride has been conducted using rat and mouse models. A study at the National Institute of Environmental Health Sciences found no deleterious reproductive effects in animals given drinking water with 100-400 times the concentration of fluoride recommended for humans.
In the early 1990s, researchers demonstrated chromosome damage in vitro in great ape cells, but not rat cells, exposed to artificially high fluoride levels.The results of this study have not been replicated in human or nonhuman primates. Fluoride critics have speculated, on the basis of this study, that humans are more sensitive than rats to fluoride. However, no evidence thus far suggests that recommended amounts have a similar effect in humans.
Fluoride and bone health. Ingested fluoride is sequestered in bone as well as teeth. The long-term effects on bone of low-level exposure to fluoride, however, are unknown. A study in Finland that examined hip fractures in elderly women who drank well water with high concentrations of fluoride found that higher fluoride levels were associated with increased risk for hip fractures in women aged 50-64 years. However, this study did not control for physical activity or other factors that affect the likelihood of hip fracture, such as small body size, smoking, and alcohol use. In a multicenter study, bone density and bone fractures in more than 9000 healthy women older than 65 years -- women who were continuously exposed to fluoridated water for at least 20 years -- experienced significant increases in bone density and a concomitant reduction in fractures.In a more recent study of elderly women living in 3 communities with varying fluoride water levels, serum fluoride concentrations were not associated with bone mineral density or osteoporotic fractures, regardless of water fluoride levels.
Fluoride and cancer. A study conducted in the 1970s, the only one to demonstrate a significant association between fluoridated water and cancer, did not control for the industrialization more prevalent in fluoridated cities. By contrast, age- and sex-controlled studies of osteosarcoma frequency and fluoridated drinking water have demonstrated no significant association. Osteosarcoma is rare and occurs most often in growing children. Still, the low frequency in this population makes it difficult to draw statistically significant conclusions about the relationship of osteosarcoma to drinking fluoridated water.