The prevalence of severe enamel fluorosis is close to zero in communities at all water fluoride concentrations below 2 mg/L. Above 2 mg/L, the prevalence rises sharply. The shape of this curve differs dramatically from the linear trend observed when all levels of fluorosis severity are combined and related to either the water fluoride concentration (Dean 1942) or the estimated daily dose in milligrams per kilogram (Fejerskov et al. 1990).
Not shown in Figure 4-1 are a prevalence of 54% in a community with a water fluoride concentration of 14 mg/L (50 Fed. Reg. 20164 ) and results from two studies of adults. One, with an age range of 20-44 years, reported prevalences of zero at <0.1 mg/L and 2% at 2.5 mg/L (Russell and Elvove 1951). In the other, with an age range of 27-65 years, the prevalences were zero at 0.7 mg/L and 76% at 3.5 mg/L (Eklund et al. 1987). These results are broadly consistent with those in Figure 4-1.
Strongly supporting evidence comes from a series of surveys conducted by researchers at the National Institute of Dental Health (Selwitz et al. 1995, 1998). In these studies using the TSIF, scores were reported only at the tooth-surface level (Figure 4-2). As with the person-level prevalence estimates (Figure 4-1), an approximate population threshold for severe enamel fluorosis is evident at water concentrations below 2 mg/L.