THE LEGISLATION establishing selective service in the U.S. armed forces, for both world wars, specified that realistic medical and cognitive criteria be instituted for drafting young males into military service. Among the medical requirements was a specification that the candidate for selective military service must possess six or more apposing, functioning teeth. The records of the Selective Service system for both World War intervals show that failure to meet this dental criterion was one of the three most frequently encountered reasons for excluding candidates from military service.

Until the 20th Century, poor dental health was considered an inevitable part of the aging process. Those whose childhoods preceded the 1960s will recall the frequency with which dental cavities (caries) impinged upon one’s childhood and how few adults had their full complement of permanent teeth. Indeed, before the 20th Century, the great majority of adults older than 30 years were either edentulous or capable of chewing food only with the aid of dentures. Since the 1960s, there has been a dramatic decline in dental caries, particularly in children, and a corresponding decline in the number of extracted teeth.

The U.S. Public Health Service attributes this happy circumstance to the widespread fluoridation of public drinking water and incorporating fluorides in commercial dentifrices. Indeed, the USPHS declares that drinking-water fluoridation “is one of 10 great public health achievements of the 20th Century.”

Fluorine, as one of the five elemental halogen gases (fluorine, chlorine, iodine, bromine and astatine), was not discovered until Karl Scheele (1742-1786) accomplished this, in 1771. The element was not isolated, however, until 1886, when the French chemist Henri Moissan (1852-1907) succeeded by overcoming fluorine’s reluctance to exist without naturally combining with other elements. Fluorine as an isolated element is highly toxic but when combined with other chemicals forms such useful commercial products as Freon (for refrigerators) and Teflon.

The relationship between fluorides in drinking water and oral health was first noted by a Colorado dentist named Frederick McKay (1874-1959), who, in 1901, noted a mottling of tooth enamel (Colorado Brown Stain) in many of his patients and ascribed it to something as yet unidentified in the drinking water. He further noted, in passing, that teeth affected by this discoloration were uniquely less susceptible to dental caries.

Only after chemists had devised spectrographic means of determining the concentration of trace elements did it become apparent that fluoride was the mysterious substance accountable both for the tooth discolorations as well as the protection against dental caries.

Could the two fluoride consequences be separable? Was there, in other words, a concentration of drinking water fluoride adequate to protect the teeth from caries while insufficient to cause the cosmetic mottling? Government scientists determined that fluoride concentrations of 0.7 to 1.2 parts per million parts of water provided protection against caries without staining the teeth.

By 1931 the government undertook an ambitious prospective program to determine whether the premise that fluoride-treated water would prevent caries was accurate. Four pairs of North American cities were selected — Grand Rapids and Muskegon, Mich.; Newburgh and Kingston, N.Y.; Evanston and Oak Park, Ill.; and Brantford and Sarnia, Ontario — for a massive study to determine the efficacy and perils of fluoridating drinking water. Fluoride, in concentrations of about one part per million, was added to the drinking water of one of each pair of cities; thus the residents of four cities drank fluoridated water and four drank water with no added fluoride. The resulting data showed no evidence supporting an association between fluoridation and any non-dental adverse condition.

The dental health of the children of these eight cities was meticulously followed for 13 to 15 years, and dental surveys showed a caries reduction of from 50 to 70 percent in the cities with fluoridation. The American Dental Association, the American Medical Association and the World Health Organization all endorsed fluoridating water supplies. Some poorer nations resorted to the less expensive alternative of adding fluorides to table salt. Water fluoridation costs vary from 31 cents per person per year for cities housing more than 50,000 persons, to $2.12 per person in very small communities. The Public Health Service estimates that fluoridation, through fluoridation of drinking water as well as of toothpastes, has saved Americans $39 billion in dental-care expenditures.

Fluorides work by inhibiting the bacteria that cause cavity-formation and by the curious action of fluorides in attracting calcium in the saliva and thus enhancing remineralization on the surface of the teeth.

Fluoridation rates vary dramatically from state to state. Hawaii is the lowest (8.4 percent) and the District of Columbia the highest (100 percent). Rhode Island, at 84.6 percent, is higher than the national average (69.2 percent). However, increased drinking of bottled water, which is rarely fluoridated, is gradually lessening the exposure of Americans to fluorides.

In a curious way, fluoridating drinking-water supplies has altered the nature and technical complexity of the dental profession from the detecting, filling or extracting of carious teeth to a substantially more sophisticated profession dealing with orthodontics, periodontia, prosthedontia (including dental implantation) and complex maxillofacial surgical interventions.

Stanley M. Aronson, M.D., a weekly contributor, is dean of medicine emeritus, Brown University ( smamd@cox.net).