Background
Community water fluoridation (CWF) is an established public health intervention for preventing dental caries, yet contemporary evidence is limited by imprecise exposure assessment. Most epidemiological studies rely on binary classifications of fluoridation status rather than quantified fluoride concentrations, which may introduce exposure misclassification. This study aimed to develop a national longitudinal database of fluoride concentrations in public drinking water supplies in Aotearoa New Zealand (NZ) and assess implications for epidemiological research.

Methods
We compiled fluoride monitoring data from all territorial authorities and regulators for 2000–2021 and linked these to spatial water distribution zones (WDZ). Data were cleaned, standardised, and aggregated to weekly concentrations. Data were integrated into an online dashboard which will be made publicly available. A synthetic cohort of 10,000 children was generated and followed from birth to age four, incorporating residential mobility and assigning WDZ-specific fluoride exposures over time.

Results
Median fluoride concentrations were 0.74 mg/L in fluoridated supplies and 0.05 mg/L in unfluoridated supplies. However, only 40% of WDZ-years maintained optimal concentrations (≥0.7 mg/L) for ≥90% of weeks, indicating temporal variability in dosing. Despite this, summary exposure measures were relatively stable across different compliance thresholds. In the synthetic cohort, children continuously exposed to fluoridated supplies had consistently high fluoride exposure (median 0.86 mg/L), while those in unfluoridated areas had minimal exposure. Children with mixed exposure showed substantial variability, with exposure strongly dependent on duration spent in fluoridated areas.

Conclusions
Exposure misclassification due to suboptimal dosing or natural fluoride variation is likely minimal in NZ. However, residential mobility introduces substantial misclassification, particularly in cross-sectional studies, potentially biasing estimates towards the null. Longitudinal, concentration-based exposure assessment is critical for improving the validity of epidemiological studies evaluating CWF effectiveness.