Long-Term Associations between Water Fluoridation and Adult Dental Caries

Dental caries in permanent teeth affects over 2 billion individuals globally. Despite declining rates among children, its cumulative impact throughout adulthood remains a challenge. The World Health Organization advocates integrating community water fluoridation (CWF) into health strategies, recognising its role in caries prevention. Recent reviews highlight the need for research examining long-term associations between CWF exposure and dental caries. The island of Ireland provides a natural comparison, as the Republic of Ireland (ROI) has fluoridated public water supplies since 1964, while Northern Ireland (NI) has not implemented CWF. This study examines associations between CWF exposure and dental caries experience in adults with near-lifetime exposure to CWF and assesses whether associations differ across tooth surfaces. Epidemiological, demographic, and sociological data collected from adults in a previous survey were analysed. Caries experience was assessed using the decayed, missing, and filled teeth (D_3CMFT) and tooth surface (D_3CMFS) indices. The primary analysis estimated associations between region (as a proxy for long-term CWF exposure) and caries experience. Negative binomial regression models, appropriate for overdispersed count data, estimated incidence rate ratios for explanatory variables, including CWF exposure, in relation to dental caries experience. Adults in the ROI exhibited significantly lower caries experience than those in NI. In adjusted analyses, individuals in the ROI had 10.6% and 15.6% lower D_3CMFT and D_3CMFS scores, respectively. The largest difference was observed on approximal surfaces (18.6%), followed by smooth (14.9%) and occlusal surfaces (11.9%). These findings support an association between long-term CWF exposure and reduced dental caries experience in adulthood. The results provide additional life-course evidence using clinical measures of disease and contribute to the ongoing evaluation of population-level fluoride policies. Alongside contemporary research, these findings may inform public health decision-making regarding CWF and its role in improving oral health outcomes in adult populations.