Objective: To estimate the total contribution of water fluoridation to caries reduction by including the benefit from the diffusion of fluoride from fluoridated communities to surrounding nonfluoridated communities via the export of bottled beverages and processed foods.
Methods: We analyzed data from the 1986-87 NIDR Children's Survey for 18,507 school children aged 6-17 years who had at least one permanent tooth and for whom a complete fluoride exposure history could be created. To measure water fluoridation exposure, we generated continuous and categorical exposure variables. Years of fluoridation exposure (YFE-continuous) measured the number of years the child lived at residences receiving fluoridated water. Lifetime fluoridation exposure (LFE-categorical) was high if the child lived at residences receiving fluoridated water more than 50% of his life and low, otherwise. We summed the proportion of state population receiving fluoridated water times the number of years the child had lived in each state and then divided this value by the child's age to measure diffusion exposure (DE). We grouped DE into three levels: low (DE<=0.25), medium (0.25<DE<0.55), and high (DE>=0.55). For each level of DE, we compared the age-adjusted mean DMFS for high and low LFE. In addition we used linear regression to measure the association between DMFS and YFE while controlling for DE, age, exposures to other fluoride sources, and sociodemographic variables. Reported results are significant at P<0.05.
Results: Comparison of mean DMFS scores found that the direct benefit of water fluoridation (DMFS(LFE=low) - DMFS(LFE=high)) was 1.44 surfaces among low DE children and 0 among high DE children. The diffused benefit (DMFS(LFE=low, DE=low) - DMFS(LFE=low, DE=high)) was 1.23 surfaces. The regression results were similar and indicated that the direct benefit would be 1.44 fewer DMFS for low DE children and the indirect benefit would be 1.09 fewer DMFS for high DE children.
Conclusion: Failure to account for the diffusion effect may result in an underestimation of the total benefit of water fluoridation, especially in high diffusion exposure regions.