Table 5 displays the sensitivity results from the combined exposures. In 32 counties, the mean quantity of natural fluoride was greater than the mean optimal level. In these cases, the added fluoride concentration was negative, which might confound any protective effect of the natural fluoride concentration seen in the analyses thus far. Therefore, these 64 county-year observations were removed and the GEE analyses re-run for both outcomes in the sensitivity analyses. For the M = 1 models, the results coincided with those presented in Tables 3 and 4. That is, added fluoride exerted a positive and significant change on incidence (0.26 per 1,000 increase) and prevalence (0.22% increase), whereas natural fluoride exhibited a protective effect (0.45 per 1,000 decrease and 0.32% decline, respectively). For the M = 2 model set, however, both exposures demonstrated negative relationships, with the effect of natural fluoride (3.12 per 1,000 decline in incidence and 2.3% reduction in prevalence) being about twice that of added fluoride (1.68 per 1,000 decrease in incidence and 1.09% decline in prevalence). Thus, the sensitivity analyses showed that only the adjusted exposures (using milligrams to account for per capita tap water consumption) revealed robustly consistent associations with diabetes outcomes.