To understand the dose-response relationship between urinary fluoride and dental fluorosis among children from drinking water-borne endemic fluorosis areas in Tianjin, so as to provide a scientific basis for assessing fluoride exposure risk among children from endemic areas and establishing reference values for urinary fluoride.

From January to December 2024, 83 endemic villages were selected in Tianjin. A total of 2 382 children aged 8-12 years from these villages underwent dental fluorosis examination, along with water fluoride and urinary fluoride testing. Additionally, data from areas where the drinking water fluorosis control target was achieved for more than 12 years (10 villages, 50 people per village) were included as controls. A restricted cubic spline model was used to analyze the relationship between urinary fluoride levels and dental fluorosis prevalence, and benchmark dose (BMD) and benchmark dose lower bound (BMDL), as well as reference dose (RfD), were calculated using the benchmark dose method.

The prevalence rates of dental fluorosis among children in drinking water-borne endemic fluorosis areas and areas of normal fluoride content in water in Tianjin were 10.58%, 7.60%, with a geometric mean urinary fluoride level of 0.72 and 0.60 mg/L, respectively. There were statistically significant differences in both dental fluorosis prevalence and geometric mean urinary fluoride levels between the drinking water-borne endemic fluorosis areas group and the areas of normal fluoride content in water (χ²/Z=4.05, -7.31, both P < 0.05). Across different periods of water source improvement, there were statistically significant differences in overall population, male, and female dental fluorosis prevalence rates and geometric mean urinary fluoride levels (χ²/H=44.95, 23.96, 21.05; 168.39, 63.93, 107.50, all P < 0.01). Significant differences were also observed across age groups among children from drinking water-borne endemic fluorosis areas in terms of dental fluorosis prevalence and geometric mean urinary fluoride levels (χ²/H=32.14, 79.73, both P < 0.01). The results of the restricted cubic spline model showed that the risk of dental fluorosis in different sex, age and overall children in the drinking water-borne endemic fluorosis areas increased significantly with rising urinary fluoride concentration(all P_{general trend} < 0.05). The BMD value for the drinking water-borne endemic fluorosis areas was 1.72 mg/L, the BMDL was 1.31 mg/L, and the RfD was 1.31 mg/L.

The prevalence of dental fluorosis among children in the drinking water-borne endemic fluorosis areas in Tianjin has been effectively controlled, but it is still higher than that in the areas of normal fluoride content in water. Moreover, urinary fluoride levels and dental fluorosis prevalence among children from drinking water-borne endemic fluorosis areas show a clear dose-response relationship.