Figure 6. Fluorine (¹⁹F) bone concentration (ppm) as a function of age.

The linear regression resulting from (A) the fluorine mean amount of 18400 to 23300 ppm measured by INAA (intercept = 11958.5±1120, P<0.001) is compared with an equivalent regression (B) obtained considering a 0 fluorine concentration at age 0 (slope = 200.4±9, P<0.001). The last model, representing the physiological rate of individual intake per year cleansed of the fraction of fluorine contamination by soil ash deposit, shows an evident age-dependent increase of fluorine (R² = 0.961). Children aged ≤10-years-old were not included in this model, due to the high diagenetic amount of fluorine released by the ash deposit. The resulting corrected mean values of 2042 to 11342 ppm show a minority of individuals matching the normal-physiological (<3500 ppm) and preclinical (<5500 ppm) ranges of fluorine bone concentration, while the majority belongs to all the three clinical phases of skeletal fluorosis, with several mature (≥40-years-old) individuals in the crippling phase III.