Prolonged ingestion of fluoride ions can cause skeletal fluorosis.

Ref. Krishnamachari, K. A. (1986). Skeletal fluorosis in humans: a review of recent progress in the understanding of the disease. Progress in food & nutrition science, 10(3-4), 279-314.

It may cause poor bone quality and calcification. The upper limit of fluoride concentration in drinking water set by the World Health Organization is 1.5 mg/L

Ref. Guidelines for drinking-water quality. 3rd ed. Geneva, Switzerland: World Health Organization; 2004.

https://books.google.co.jp/books?hl=en&lr=&id=SJ76COTm-nQC&oi=fnd&pg=PR15&ots=V8tZo9N4Ze&sig=no2cnwpX1JLdCwTMLfMjxggfKho&redir_esc=y#v=onepage&q&f=false

Enamel fluorosis is a developmental disturbance of enamel formation due to excessive F ingestion during amelogenesis. This exerts a direct inhibitory effect on enzymatic functions of the ameloblasts, resulting in defective matrix formation as well as subsequent hypomineralization. Enamel formation of permanent teeth, other than third molars, occurs from the time of birth until approximately five years of age and completely formed at age of 8 years. After enamel is completely formed, dental fluorosis cannot develop even if excessive F is ingested. Bone fluorosis results from long, continued exposure to excessive amounts of F in drinking water. Bone has a high tolerance for fluoride without showing clinical and pathologic manifestations. The earliest radiographic evidence of bone fluorosis may be detected in about 10% of the adult population who ingested throughout their lifetime, water containing fluoride levels of at least 8 ppm.

This occur during intra utriene life before tooth developement