380 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS relative number of randomly distributed disulfide bonds in the fiber. This technique was used to determine the comparative swelling rates of beard and scalp hair. In 97% formic acid the volume swelling (Vi•) of Caucasian beard and scalp hair was determined to be 0.41 + 0.02 and 0.44 _+ 0.02, respectively. Since Vi• was defined as the volume of dry hair expressed as a fraction of the volume of swollen hair, the lower volume swelling index for beard hair indicates a higher degree of swelling and thus a lower degree of disulfide bonding. In good agreement with the above studies, Caucasian beard fibers exhibited a faster rate as well as a higher degree of swelling in 97% formic acid, than scalp hair of similar diameter (Figure 17). 7O ojø .m E .e_ 20 10 I I I I i I 2 3 4 5 Time (minutes) Figure 17. Diametral swelling of Caucasian beard and scalp hair in 97% formic acid solution. Each data point is the average of measurements from five fibers for each of five subjects (25 samples). d. Permanent Set and Supercontraction Supercontraction and set are characteristics of keratin fibers in certain media (22), and their level is principally dependent on the chemical composition of the cortical regions of the fibers (23). As fiber thickness is another parameter influencing the level of set incurred (23), facial and scalp hairs of similar diameters were selected for this study.

COMPARISON OF BEARD AND SCALP HAIR 381 Table VII Permanent Set and Supercontraction of Caucasian Beard and Scalp Hair Property Beard Hair • Scalp Hair b Permanent Set, (%) 15.7 _+ 0.9 12.7 + 1.1 Supercontraction, (%) 25.7 + 1.1 19.0 + 1.5 aBulk hair from ten subjects. bBulk hair from De Meo Brothers. Values are averages of four determinations along with standard deviations. In boiling sodium bisulfite, a disulfide bond cleaving agent, the beard hair attained higher levels of permanent set and supercontraction than the scalp hair (Table VII). The results indicate that the facial hair, having a lower level of stabilizing disulfide links, is more susceptible to molecular rearrangements. CONCLUSIONS A comparative study of the morphology, chemical composition, mechanical properties, and chemical reactivity revealed significant differences between scalp hair and facial hair. Morphologically, differences were found between beard and scalp hair in cross- sectional'area and shape, number of cuticle layers, cuticular pattern and medullation, in all three ethnic groups investigated. In addition, racial origin leads to minor variations in fiber size, geometry, and pigmentation. Chemically, the most significant difference between the two fiber types was the lower disulfide bond density in beard hair, as indicated by its lower cystine content. The lower elastic modulus, faster swelling rate, and higher reaction rate of beard hair with a variety of reagents was the result of the less extensive disulfide bonding in this fiber than in scalp hair. REFERENCES (1) J. A. Swift, New developments in electron microscopy,J. $oc. Cosmet. Chem., 22,477-486 (1971). (2) R. Dawber and S. Camaish, Scanning electron microscopy of normal and abnormal hair shafts, Arch. Dermatol., 101,316-323 (1970). (3) J. A. Swift and A. C. Brown, The critical determination of fine changes in the surface architecture of human hair due to cosmetic treatment,J. Soc. Cosmet. Chem., 23,695-702 (1972). (4) N.H. Leon, Structural aspects of keratin fibers,J. Soc. Cosmet. Chem., 23,427-445 (1972). (5) L.J. Wolfram and M. Lindemann, Some observations on the hair cuticle, J. Soc. Cosmet. Chem., 22, 839-850 (1971). (6) J. Menkart, L.J. Wolfram and I. Mao, Caucasian hair, Negro hair and wool: Similarities and differences,J. Soc. Cosmet. Chem., 17, 769-787 (1966). (7) R. A. Wall and L. R. D. Hunter, Normal adult hair--structure and properties, Cosmetics and Perfumery, 89 (2), 31-36 (1974). (8) R. C. Clay, K. Cook, andJ. I. Routh, Studies in the composition of human hair,J. Am. Chem. Soc., 62, 2709-2710 (1940). (9) J. A. Swift and B. Bews, The chemistry of human hair cuticle. I. A new method for the physical isolation of cuticle,J. Soc. Cosmet. Chem., 25, 13-22 (1974). (10) J. F. Corbett, The chemistry of hair care products,J. Soc. Dyers Colour., 92, 285-303 (1976). (11) J. A. Swift, Chemical composition of various morphological components isolated from human hair cuticle, Cosmetics and Toiletries, 91, 46-48 (July 1976).