Assimilation of fluoride from water into the bloodstream in humans can be seemingly well-tolerated for long time periods because bone efficiently traps the calcium chelator from interstitial fluid to minimize exposure to soft tissues. Bone is the final resting site for 95% of all retained fluoride that causes increased osteoblast activity in response to the intrusion.
Fluoride accumulates from consumption in a 1 ppm fluoride water region, in the absence of other known sources, to 2,500 mg/kg in approximately twenty years and to 3-4,000 mg/kg lifetime. Bone weakening occurs during this process which can also include bone pain. Some cases of bone pain have been described at bone fluoride levels as low as 1,700 mg/kg. Before bone weakening occurs, the abnormal incorporation of fluoride causes the conversion of hydroxyapatite to fluoroapatite with a different crystal structure, and formation of poor quality bone.
As more and more fluoride is consumed, the accumulation of fluoride in bone continues to grow. In certain regions of the world such as in the Middle East, where fluoride levels in water are naturally relatively higher than targeted levels in fluoridation, the final endpoint of lifelong accumulation of fluoride in bone is commonly reached.
A severely fluorotic human leg bone in a victim of stage III fluorosis is pictured here with a normal human leg bone. The process of fluoridation of bone begins when water with fluoride begins to be consumed.