What Atharva and Ericmuch say is sometimes contrary to the truth. Calcium fluoride cannot be dangerous, because it is insoluble in water, and in diluted acidic solutions. It can produce negative effects such as skeletal fluorosis, but this happens only in cases where it is taken in huge amounts. Fluorides have no negative effects, if taken in small amounts.
It is even the contrary. Adding sodium fluoride in small amounts (90 - 250 mg/kg) to the usual salt NaCl has had a remarkable effect on the dental health of the general population. I can give you some values measured before and after the introduction of NaF in the salt (or in the tap water). This introduction has been introduced in USA during the war 1939 - 1945. It was adopted much later in Europe, where it was accepted with hesitation and reluctance.
I can give the results I know from my own country, Switzerland. In 1961, before the fluoration in NaCl, only 25.2% of the 7y children had no dental caries (tooth decay). And only 1.2% of the teenagers (15 y) had no dental caries. And an average teenager had 7.4 teeth with caries.
The fluoration of the salt was introduced in 1962. Fifteen years later, in 1977, 76.6% of the young children (7 y) had no dental caries at all. The tendency has been reversed. And 15,1% of the teenagers (15 y) had no dental caries, instead of 1.2% earlier. And the average teenager had 1.2 teeth with caries. It is a huge progress. It was not perfect, of course, but it shows that adding a small amount of fluorine improves the dental health.
The same measurements have been made in many if not all European and American countries. See the following references.
H. Dean, Fluorine in the Control of Dental Caries, Intern. Dent. Journal 4: 311-337,, 1954.
K. Binder, W.S. Driscoll, Caries-Preventive Fluoride Tablets Programs, Car. Res. 12 (Suppl. 1):22-30, 1978.
F.B. Glenn, The Rationale for the Administration of a NaF supplement, J. Dent. Child, 48:118-122, 1981.
I. Hellsröm, Y. Ericsson, Fluoride Reactions with Dental Enamel Following Different Forms of Fluoride Supply, Scand. J. Dent. Res. 84:225-267, 1976.
H.R. Mühlemann, K.G. König, T.M. Marthaler, Anleitung zur praktischen individuellen Kariesprophylaxie, Zahnärtl. Mitt, 16, 741 -743 1966. H.S. Horowitz et al., Perspectives on the use of Prenatal fluorides : A Symposium, J. Dent. Child 48, 102-134, 1981. T.M. Marthaler, Ph. de Crousaz, Médecine dentaire préventive, en particulier dans les écoles, Schweiz Mntschr. Zahnheilk., 89: 619-628, 1979. K. Toth, Ten Years of Domestic Salt Fluoridation in Hungary, Caries Res. 13:101, 1979.