LIGHT RADIATION EFFECTS ON HAIR 673 Table III Shielded Control Group Exposure Period: 6-29-70 to 9-8-70 Hair No. Yield Point at 15% Strain (g) 1st Test 2nd Test •o Change s 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 29.0 27.4 31.0 25.6 36.0 38.0 3O 0 3O 7 28 4 2O 3 28 6 33 2 29 3 29 5 32 3 37 6 37 4 25 2 28 1 32 9 36 0 28 7 35 9 35 0 34 5 27 8 26 7 30 0 24 9 34 9 36 7 29 1 30 0 27 5 19 9 27 8 32 3 28 7 28 7 31 2 36 1 36 2 24 6 27 1 32.2 35.0 27.6 34.9 33.8 33.7 --, .13 --2 55 --3 22 --2 73 -- 05 --3 42 --3 00 --2 28 --3 16 --1 97 --2 79 --2 71 --2.04 --2.71 --3.40 --3.98 --3.20 --2.38 --3.55 --2.12 --2.77 --3.83 --2.78 --3.42 --2.31 Average, --2.95% std. dev., -v0.60. These yield point data indicate the tensile effect of summer solar radiation to be five times greater than that which was experienced during the winter months. While the differences in the seasonal radiation levels were expected, the pronounced tensile alteration and the effect on the disulfide linkages in the hair keratin were not anticipated. The shielded sample groups experienced minimal changes during the test period and were probably due to residual infrared scattering or thermal radiation. The bleach-damaged hair, with fewer disulfide linkages (17), was included in this study to determine whether or not the additional radiation damage was additive or potentJared. These results indicate an additive effect at these low levels of bleach damage. The data in Tables III and IV illus- trate the yield point measurements, as grams of force at 15% strain, of an exposed and shielded sample group of hair from a summer test.

674 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table IV Exposed Sample Group Exposure Period: 6-29-70 to 9-8-70 Yield Point at 15% Strain (g) Hair No. 1st Test 2nd Test % Change s l 2 3 4 5 6 7 8 9 l0 ll 12 13 14 15 16 17 18 19 20 21 22 23 24 25 30.8 28.0 35.0 27.7 32.6 25.1 22.2 24.2 29.6 38.5 31.7 28.4 35 7 30 0 34 0 29 7 33 7 28 6 28 2 31 6 25 1 27 5 25 1 31 2 34 2 16.5 14.7 20.5 14.0 19.2 12.4 12.0 12.1 16.0 22.8 18.4 15.4 21.1 17.4 20,2 16.5 20.6 15.6 15.2 18.3 11.6 14.6 12.4 16.6 19.6 --46 42 --47 50 -- 41 42 -- 49 45 --41 10 --50 59 --45 94 --50 00 --45 94 --40 77 --41 95 -- 45 77 --40 89 --42 00 --40.58 -- 44.44 -- 38.87 --45.45 -- 46.09 -- 42.08 --53.78 --46.90 --50.59 --46.79 --42.69 Average, --45.13% std. dev., -4-3.85. The changes in the tensile properties of hair are also apparent in the UV irradiation experiments whose data are reported in Tables V and VI. As in the sunlight experiments, the change in the yield point at 15% elongation is reported as the average for a 25-specimen sample group. The shielded sample groups also exhibited minimal changes in the ten- sile properties of the hair while the results of the bleached-damaged hair indicated additive damage as in the sunlight tests. The increased effect on the tensile properties with increased amounts of radiation disclosed an exponential relationship which is illustrated in Fig. 4. Previous work with radiation-induced damage to macromole- cules revealed a logarithmic function (18-20). This relationship was explained by the decreasing probability of radiation striking the disul- fide bonds as the dose increased. However, our test results suggest the