Figure 1: (a) Calcium concentrations (mM) that precipitate various fluoride concentrations. From the known solubility product constant Ksp for calcium fluoride, the calculated isocurve for CaF2 precipitation is shown as a function of varying concentrations for the two ions. Calcium levels in normal blood (1–3 mM) reach precipitation maximum solubility with 0.1–0.18 mM (1.9–3.4 ppm) fluoride. For 3 mM calcium, precipitation begins to occur when fluoride is 3% of calcium and for 1 mM at 18%. The Hooper Bay poisoning incident produced a lethal 5 ppm blood fluoride level in one victim. Whether it is a low Ca/F ratio or rather calcium fluoride saturation that is required for acute toxicity is not known. (b) Calcium effects on measured fluoride ion concentration. A 0.9 ppm fluoride solution in distilled water was measured for free fluoride ion level with a LaMotte fluoride specific electrode calibrated with 1.00 ppm sodium fluoride in distilled deionized water at room temperature. Calcium ion was adjusted over a wide range by addition of aliquots of calcium biphosphate. Fluoride readings progressively decrease with increasing calcium concentration as expected over the range 30–650 mM.