Longitudinal Assessment of Fluoride Concentrations in Public Drinking Water Supplies in New Zealand: Implications for Epidemiological Studies

OBJECTIVES: To establish a national longitudinal database of fluoride concentrations in public drinking water supplies in New Zealand (NZ) and investigate the potential implications of different fluoride exposure classifications for epidemiological studies evaluating the effects of community water fluoridation (CWF) on population health outcomes. METHODS: Fluoride concentrations in NZ public drinking water supplies were assessed longitudinally from 2000 to 2021. Fluoride sample results obtained from all 67 Territorial Authorities and drinking water regulators were cleaned, standardised to water distribution zones (WDZs), aggregated to weekly values, and imputed where missing. A synthetic birth cohort of 10 000 children followed from birth to age four was developed to simulate individual-level fluoride exposure histories and evaluate different exposure classifications, including binary fluoridation status and mixed exposure resulting from residential mobility. RESULTS: Fluoride concentrations in fluoridated WDZ had a median of 0.74 mg/L (range 0.00-1.04), while non-fluoridated WDZ had consistently low fluoride concentrations (median 0.05 mg/L; range 0.00-0.42). Substantial temporal variability was observed, with only 40% of WDZ-observation years having fluoride concentrations above 0.7 mg/L for at least 90% of the year. In the synthetic cohort, exposure misclassification arising from naturally occurring fluoride concentrations or sub-optimal dosing appeared minimal. In contrast, residential mobility between fluoridated and non-fluoridated areas represented a more substantial source of potential bias, with approximately 30% of individuals experiencing mixed fluoridation exposure over 4 years. CONCLUSION: The findings highlight the importance of longitudinal and quantitatively derived exposure measures for future research evaluating the oral health impacts of CWF.