Optimal Fluoride Concentration in Drinking Water for Preventing Dental Fluorosis and Caries in 8-12-Year-Old Children in Qingdao, China: A Restricted Cubic Spline and Spatial Epidemiological Analysis Following Rural Safe Drinking Water Interventions in Endemic Fluorosis Region.

Maintaining optimal fluoride levels in drinking water is crucial for balancing dental caries prevention and fluorosis risk in children. This cross-sectional study investigated 12,163 children aged 8-12 years across 934 villages in Qingdao, China, following water safety interventions in an endemic fluorosis regions. Water fluoride concentrations were measured via spectrophotometry, while dental examinations assessed fluorosis (Dean's classification) and caries (DMFT index). Spatial epidemiological mapping, restricted cubic spline (RCS) models, and multivariate logistic regression were integrated to analyze geographic patterns, dose-response relationships, and risk thresholds. Results demonstrated an overall fluorosis prevalence of 3.2% and caries prevalence of 27.0%. RCS analysis revealed a nonlinear positive association between fluoride levels and fluorosis risk (threshold: 0.70 mg/L), alongside a U-shaped caries-fluoride relationship exhibiting minimal caries risk at 0.70-0.73 mg/L. Multivariate regression identified significantly elevated fluorosis odds (adjusted OR = 1.71, 95%CI 1.29 - 2.28) at concentrations > 0.85 mg/L, while 0.70-0.73 mg/L optimally minimized caries. Spatial clustering of high-fluoride regions in northwestern Qingdao correlated with fluorosis hotspots. These findings establish 0.70-0.73 mg/L as the optimal fluoride concentration range for dual caries prevention and fluorosis mitigation in this population. The study advocates for dynamic adjustments to water fluoridation standards and geographically targeted interventions in high-risk regions, offering evidence-based strategies for public health policymaking in fluorosis-endemic regions.