- 1 part per million fluoride (1 ppm F, 1 mg/L) is equivalent to 52 µmol/L
1 µmol/L is 0.019 ppm fluoride.
DenBesten P, Li W - "Chronic fluoride toxicity: dental fluorosis" Monogr Oral Sci 22:81-96 (2011)
================================================================================="In humans, plasma fluoride concentrations resulting from long-term ingestion of 1–10 ppm fluoride in the drinking water range from 1 to 10 μmol/l. Fluorotic changes can be obtained in incisors of rodents drinking water containing 25–100 ppm fluoride; these doses also elevate plasma fluoride levels to 3-10 μmol/l, similar to those found to cause fluorosis in humans. A complicating factor in assessing the exact dose, or determining the stages of enamel formation most sensitive to fluoride, is that fluoride incorporated into bone is gradually released by continuous bone remodeling [5, 8]. Levels of plasma fluoride as low as 1.5 μmol/l (resulting from fluoride release from bone) are still capable of inducing mild enamel fluorosis in the rat incisor after the initial exposure ends [4, 8]."
Bronckers AL, Lyaruu DM, DenBesten PK - "The impact of fluoride on ameloblasts and the mechanisms of enamel fluorosis" J Dent Res 88(10):877-93 (2009)
"In drinking water, fluoride levels of at least 10-30 ppm are necessary to induce lasting enamel disturbances in rodents, and levels used in most experimental studies are in the range of 25-100 ppm (Shinoda, 1975; Angmar-Månsson et al., 1976; Fejerskov et al., 1979; Ekstrand et al., 1981; Angmar-Månsson and Whitford, 1984; DenBesten, 1986; Kubota et al., 2005)."