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."
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)."