Relationships between Toenail, Urinary, and Drinking-water Fluoride Concentrations in a Pregnancy Cohort using Private Water Systems in the United States

To assess the strength of the relationships between fluoride concentrations in tap water, toenails, and urine in a U.S. cohort of pregnant participants using private, unregulated water systems. We analyzed fluoride concentrations in residential tap water directly with a fluoride-specific electrode, and in toenail and spot urine samples using a micro-diffusion technique. We collected samples from 436 pregnant women in the New Hampshire Birth Cohort Study. We used Spearman's rank correlation to assess relationships between fluoride concentrations in water, urine and toenails, stratified by self-reported percent of residential tap water use and residential water filtration practices. In sensitivity analyses, we re-assessed these relationships by excluding participants who reported tea consumption during pregnancy. Median (IQR) fluoride concentrations were 0.19 (0.38) mg/L for tap water, 2.60 (1.71) µg/g for toenails, and 0.39 (0.38) mg/L for specific gravity-adjusted urine. For 18% of participants, tap water fluoride concentrations exceeded the U.S. Public Health Service recommended 0.7 mg/L. Fluoride concentration in tap water was correlated with that of toenails (rho = 0.31, p < 0.01) and urine (rho = 0.18, p < 0.01). These correlations were stronger for participants reporting higher household tap water use and no residential filtration. After excluding tea drinkers, the water-urinary fluoride correlation increased more markedly than the correlation between water and toenail fluoride. Fluoride in tap water was positively correlated with urinary and toenail fluoride levels among pregnant participants, with the overall strongest correlation observed for toenails. Our findings support the potential of toenail fluoride as a reliable long-term biomarker for assessing fluoride exposure during pregnancy.