Fluoride in Drinking Water: We estimate a zero effect on cognitive ability in contrast to several recent debated epidemiological studies

The Effects of Fluoride in Drinking Water. Linuz Aggeborn and Mattias Öhman. Journal of Political Economy, Jan 2021. https://www.journals.uchicago.edu/doi/full/10.1086/711915

Abstract: Water fluoridation is a common but debated public policy. In this paper, we use Swedish registry data to study the causal effects of fluoride in drinking water. We exploit exogenous variation in natural fluoride stemming from variation in geological characteristics at water sources to identify its effects. First, we reconfirm the long-established positive effect of fluoride on dental health. Second, we estimate a zero effect on cognitive ability in contrast to several recent debated epidemiological studies. Third, fluoride is furthermore found to increase labor income. This effect is foremost driven by individuals from a lower socioeconomic background.

VI.  Discussion and Conclusion

Let us now return to our findings on cognitive ability. We claim that we find no effect of fluoride on cognitive ability, but is the estimated effect effectively zero? Let us monetize the estimates by relating them to earlier published findings on the predicted power of cognitive ability. We then choose column 5 in Table 4, where fixed effects and covariates are included. Our point estimate is 0.0028, with fixed effects and covariates included, for an increase of 0.1 milligrams/liter of fluoride on cognitive ability.

Lindqvist and Vestman (2011) estimate the return of cognitive ability on wages using Swedish registry data. Let us do a back-of-the-envelope calculation. Their results in Table 1 indicate that a 1 standard deviation increase in cognitive ability yields an approximately 10.4% increase in wages. We multiply their return to cognitive ability with our results for the effect of fluoride on cognitive ability. The estimated effect of an increase of 1 milligram/liter of fluoride translates to an 0.29% increase in wages.19 In conclusion, the close to zero and insignificant result that we estimate for the effect of fluoride on cognitive ability translates to a small impact on wages.

Another way to evaluate a zero result is to look at earlier studies that have found statistically significant results and compare the precision of the estimates. Our study includes more than 80,000 individuals when we do not include covariates or fixed effects and about 47,000 individuals with covariates and fixed effects. This may be compared with Green et al. (2019), which included around 600 observations, and the reviewed studies in Choi et al. (2012), where the number of observations was less than 1,000 for the largest study. Our confidence intervals are tighter than the 95% confidence intervals in all earlier studies.20

The remaining question is why our results deviate from previous studies, such as Green et al. (2019), that have considered similar fluoride levels.21 The main objection against Green et al. (2019) is that the choice of fluoridating water is an endogenous policy variable. Individuals do not exogenously live in fluoridated areas, making it likely that there are selection problems present. It is also noteworthy that Green et al. (2019) find a negative association only for boys and not for girls. However, we should note that Green et al. (2019) have access to urine data with actual fluoride measures within the body and several background variables that we do not have access to and that they also measured IQ at a younger age than we do.

Our results are policy relevant for developed countries with water fluoridation, given that water authorities seldom consider fluoridation above 1.5 milligrams/liter. How do our results relate to developing countries in terms of external validity? We have no reason to expect that the effect of fluoride on cognitive ability is dependent on the institutional setting. Fluoride is a chemical substance, and its effect on cognitive development should not be specific to Sweden. Choi et al. (2012) consider studies from China and Iran with fluoride levels similar to ours but also studies with higher levels, and they concluded an overall negative association. Although the mass of fluoride is within the range of 0–1.5 milligrams/liter in our data, we have some observations above the 1.5 milligrams/liter threshold set by the World Health Organization. The share of observations in this upper limit is still large in comparison to the studies reviewed in Choi et al. (2012). Figure A4 and table A7 focus on these high-level treatment effects and display no evidence of a negative effect of fluoride up to at least 3 milligrams/liter. These results should be interpreted with caution given that it is a selected sample, but it covers many of the papers in Choi et al. (2012) in terms of range. Given that our results deviate from studies reviewed in Choi et al. (2012), we believe that many of the studies capture other simultaneous hazardous treatments.

Our paper is about not only cognitive ability but also the effect of fluoride on dental health and income. Regarding dental health, we believe that our results are generalizable. Fluoride does improve dental health, and our natural experiment confirms this well-established finding in a long-term setting. However, we should remember that we measure dental health indirectly through the dental health care system in Sweden, with a large supply of dental care. The outcome where we expect to have the least external validity is our income measure, where the mechanism channels previously discussed are dependent on the institutional setting. It is interesting to note that our estimates on income, derived from rich and detailed population-wide data, are in line with Glied and Neidell (2010), who used American data.

Our findings add to the literature on the effects of fluoride on cognitive ability, but we have also broadened the understanding of the effects of fluoride by studying dental health (the first-stage relationship) and income (the long-term outcome). On the basis of the results, fluoride exposure through drinking water seems to be a good mean of improving dental health without negative effects on cognitive development for the fluoride levels considered in this study.