Deficiency of F− is much less than 1 ppm in drinking water causes(I) tooth decay(II) brown mottling of tooth(III) methemoglobinemia

F− concentration close to 1 ppm strengthens teeth by converting its basic hydroxylapatite layer [3 Ca3 (PO4)2] to much harder fluorapatite 3 Ca3 (PO4)2 . CaF2.  F− concentration close to 2 ppm causes brown mottling of teeth while close to 10 ppm weakens bones.