is reduced with increasing water hardness Seawater contains fluoride at a concentration of 1.3 mg/L. Mechanism Like most soluble materials, fluoride compounds are readily absorbed by the stomach and intestines, and excreted through the urine. Urine tests have been used to ascertain rates of excretion in order to set upper limits in exposure to fluoride compounds and associated detrimental health effects. Ingested fluoride initially acts locally on the intestinal mucosa, where it forms hydrofluoric acid in the stomach. References External links Element toxicology Toxic effects of substances chiefly nonmedicinal as to source Fluorides