380 JOURNAL OF COSMETIC SCIENCE Raman measurements. Raman measurements were performed on a Nicolet 950 Raman spectrometer using the 1064-nm light beam of the Nd+3-YAG CW laser at a power level of-90 roW. The beam diameter was ca. 1 min. Data were collected at a nominal resolution of 8 cm -1. Each run consisted of 1000 co-added scans. Two measurements were made on the exposed portion of each tress and two on the unexposed area. The data were averaged to obtain the representative spectra for each tress. The averaged spectra for a tress were then combined with the data similarly obtained for the other tress to produce average spectra for a given treatment. The background fluorescence was re- moved using the OMNIC © baseline routine. The spectra were then normalized using 1450 cm- 1 methylene stretch, considered invariant, using the OMNIC © library routine. The characteristic features of the Raman spectra, thus obtained, have been discussed previously (12). It should be noted that the measurement sites on the exposed and the unexposed areas were spatially separated by no more than 2 cm to keep to a minimum the differences between these regions due to natural weathering. This was confirmed by Raman measurements prior to photo-exposure, which showed no measurable difference between these sites. The amount of damage was then calculated from the changes in the intensity of the band at ca. 510 cm -1 due to S--S stretching, as follows: % Damage = [(I(unexposed ) - I(exposed))/l(unexposed)] X 100 (5) Reflectance measurements. Measurements were made using a Perkin-Elmer Lambda 6 in- strument connected to a Labsphere integrating sphere. Hair tresses were measured directly in the reflectance mode and converted into absorbance units by the data manager software. RESULTS AND DISCUSSION The effect of sunlight on the mechanical strength of the pigmented and the unpig- mented hair fibers is compared in Figure 1. The data clearly reveal that the mechanical strength of the unpigmented hair is compromised at a significantly faster rate than that of the natural brown hair. Since the diameter of the pigmented hair fibers was only ca. 5 % larger on average, compared to the unpigmented fibers, the difference in the pho- todamage rate represents the photoprotection afforded by the melanin pigment. The photoprotective aspect of melanin has also been described previously in the literature (5,8,9). Dyeing of hair results in the deposition of color on the cuticles and, depending on the product, a significant amount may also penetrate all the way inside the fibers. In contrast, the natural color is localized in the cortex. Barring any secondary effects such as photochemical reactions, we would expect the hair color to provide protection against sun damage by partially absorbing and thereby attenuating the incident light. Figure 2 shows the effect of permanent hair coloring on the photodamage rate of hair fibers. Notice that the dyed fibers are initially (no photo-exposure) weaker than the control fibers due to oxidative damage to hair during the coloring process. Photoirradiation affects the colored and the control fibers differently the colored fibers are damaged less than the undyed control fibers at each irradiation level. The net effect of this difference in the rate of damage is that beyond a characteristic irradiation time, where the two lines intersect, the dyed fibers are stronger than the control. This crossover time, 7o is

HAIR PHOTOPROTECTION BY DYES 381 20 air 0 3 6 9 Photoexposure (days) Figure 1. Photodamage of natural brown and Piedmont (pale yellow) hair as a function of photo-exposure time. The solid lines represent the best linear fit to each data set. The slopes indicate the rate of light- induced damage. characteristic of a dye product, as seen in Table I. Typically, the darker the color (higher dye load) the shorter the x c, or higher the degree of photoprotection, all else being the same. The demipermanent, or tone-on-tone hair colors, in comparison to the permanent colors, are applied for a shorter duration, have lower alkali and peroxide concentrations and, therefore, produce less initial chemical damage. Figure 3 compares the effect of photo- irradiation on Piedmont hair dyed with the black shade of a demipermanent color with the undyed control. Notice the lower initial chemical damage and a shorter x c compared to the permanent color (Figure 2). The effect of light alone, extracted from the composite of chemical- and light-induced damage shown in the above-mentioned two figures, is depicted in Figure 4. For com- parison, we have also included the data obtained for the undyed hair. The slopes of the curves are proportional to the rate of damage: the larger the slope, the less the protection. These data clearly reveal that the permanent color is more efficacious than the demi- permanent when applied to unpigmented fibers. This is likely due to the fact that since the permanent coloring products are left on the hair for 25 minutes, compared to ten minutes for the demipermanent products, they deposit more dyes on the hair. This higher dye deposit, in turn, leads to a higher degree of photoprotection. The data discussed above show that photodamage is linear with time. Although these represent the black shades, similar results were obtained with other shades as well. This would suggest that the efficacy of the dyes in protecting hair does not measurably diminish with time. This is somewhat surprising in view of the fact that a significant amount of dye fades during our experimental conditions, as seen in Figure 5 for the red