… In China, endemic fluorosis is classified into drinking-water type, drinking-tea type, and coal-burning type. Humans are exposed to high fluoride mainly through drinking groundwater with high fluoride content (Barbier et al., 2010). When fluorine is absorbed, it enters the blood and distributes throughout the body, mainly in bones and teeth (Barbier et al., 2010). In addition, 50–80% inhaled fluoride can be excreted through the urinary system, and thus, urine fluoride concentration as an internal exposure index can systematically reflect the burden of fluoride exposure in drinking water (Ding et al., 2011).
… We also determined a safety threshold of urine fluoride on intelligence impairment in the subgroup TT as 1.73 mg/L urine fluoride with a 95% CI of (1.51 mg/L, 1.97 mg/L). According to WHO Drinking water Quality 4th edition (2011), fluoride in drinking water quality standard is 1.5 mg/L (WHO, 2011). In China, the water fluoride limits for large centralized and small centralized or decentralized water supplies are 1.0 mg/L and 1.2 mg/L, respectively (Liu et al., 2016). However,
these standards were set based on both drink water fluoride levels and general health significance. To date, no national or international intelligence impairment-based safety threshold of urine fluoride levels (an internal exposure index) is available. Thus, the IQ-based safety threshold of urine fluoride levels that we identified in the subgroup TT may be considered as an initial testing value to be refined at multilevels of the laboratory, epidemiological, and clinical studies.
Strengths of our study include using urine fluoride as an internal exposure index and thus minimizing the measurement error of exposure, adjusting up to 30 potential confounding covariates including child age and gene polymorphismin regressing IQ on urine fluoride in children, and careful modeling with applications of cross-validation, bootstrap techniques, and sensitivity analysis. Meanwhile, we were also limited by a cross-sectional study design, no data on breastfeeding which might influence both urine fluoride levels and IQ scores in children, possible residual confounding of covariates, and relatively small sample size of the subgroup TT.