FLUORINE AND DENTAL CARIES 191 Some contrary opinions have been expressed. Bunting in 1928 reported (13) his finding that the frequency of caries was the same although the extent and activity of caries was less in fluoride areas. Applebaum in 1936 reported (2) that he found a lack of correlation between the severity of mottling and the number of carious areas. M. C. Smith (53) warned that the favorable results might be misleading. Smith reported that in St. David, Ariz., where the drinking water contained from 1.6 to 4.0 ppm. of fluorine, children of 12-14 years did show a definite rec•uction in caries but that residents aged 2141 showed excessive caries. Weaver (58) also stated that fluorine had a postponing action, but did not prevent caries, the delay not exceeding five years. FLUORIDE ADDED TO DIET In sp{te of the few contrary op{n{ons, and in spite of poss{b]e dangers which will be discussed later, there have been numerous proposals to add fluoride inten- tionally to the diet (usually to the drinking water) in order to reduce caries. Tests are at present be{ng con- ducted in Evanston, IiL, Grand Rapids, Mich., and Newburgh, N. Y., in which fluorine is being pur- posely added in low concentration to the drinking water of these test towns, while neighboring control towns are continuing to use fluoride- free water. The results of these tests, over a sufficiently long period, should be convincing. Such results presumably cannot be entirely conclusive, however, until continued for the entire life span of the generation born after the experiments started. The earlier studies, both on animals and on humans, indicated that the caries-inhibiting and fluorosis-producing action of fluoride-containing drinking water were both found when the water was consumed during the tooth formative age (1). It was naturally assumed that the tooth enamel contained fluorine, which, as a result, and de- pending upon the amount of fluorine laid down in the enamel, showed an increased caries resistance and fluo- rosis or mottling. ' Continued observation has indi- cated that the consumption of fluorine-containing water may still have a caries inhibiting effect on fully erupted teeth (25, 39). Since it is unlikely that ingested fluorine can be deposited by meta- bolic processes in the enamel of erupted teeth, these observations have suggested that fluorine may find its way directly from drinking water into the tooth structure. Volker, in 1939, showed (56) that tooth enamel would pick up fluoride on contact with fluoride solution. TOPICAL APPLICATION OF FLUORIDE SOLUTIONS There have been numerous re- ports of the use of fluoride solutions by topical application, recognized by a recent editorial in the yournal

192 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS of the •lmerican Dental •lssociation (1) which recognizes the probable effectiveness of such procedure but warns of potential dangers. Armstrong was reported in a newspaper article in November, 1940, as believing that topical application of fluoride would prevent caries. Bibby (10) reports on a new technique by which dental prophy- laxis is conducted with 1 per cent NaF, H202, and pumice, and he claims that two prophylaxes will reduce caries by 25 per cent while three prophylaxes will reduce caries by 43 per cent. Cheyne (15, 16) found that topical applications of solutions of from 500 to 60,000 ppm. of fluorine were useful in reducing carious activity in existing lesions and d•- creased the formation of n•w lesions. Jordan (38) and Knutson (40, 41, 42) reported that a solution of 2 per cent NaF, applied topically, would prevent new carious areas, but would not stop the carious process if previously initiated. An editorial in the British Medical yourhal (29) and a review by Hodge (34) both acknowledge the effectiveness of topical application, while further work by several other workers verifies the effectiveness of local application. These results all lead to a con- clusion that the topical application of fluoride has some action in pre- venting caries. This conclusion cannot be recorded at this point, however, without adding that in the writer's opinion, all the positive evidence indicates that repeated applications of preparations of very high fluoride content are required, and that there is, as yet, no con- vincing evidence that any signifi- cant value can be expected from the addition of reasonably low fluoride concentrations to denti- frices. Before entering upon a further discussion of the mechanism of fluoride action it is necessary to outline, as a working hypothesis, a mechanism of caries itself. Caries is regarded as a condition attacking the tooth from an outside surf act. The original attack of caries upon tooth structure is regarded by many as a simple chemical attack by acids. In order for the acids to produce the typical localized attack it is assumed that they form, and are concentrated within a dental plaque•an adhering, discrete sur- face deposit or a food particle wedged into a narrow space such as a crevice or an interproximal space, which holds the acids in situ in spite of rinsing of the surface "by water or saliva. The acids are presumably formed by living organisms acting within the plaque or particle on carbo- hydrates originating in foods. The organisms are acidogenic in nature and comprise one or more of the group of Bacillus acidolphilus, acidogenic Streptococci, yeasts, and molds. The carbohydrate substrate is typified by sugar, but other sub- stances may also be involved.