JOURNAL OF COSMETIC SCIENCE 642 GEOMETRIC PROPERTY CHANGE BY AGING In Figure 4, the hair diameter (minor axis) is plotted against age. These are the averaged values of all hair fi bers sampled from a 7-mm × 7-mm area of the top of the head. Here, the hairs less than 5 cm in length are avoided so as not to count the newborn hairs. The hair diameter increases until the age of around 40, then decreases. This trend is consis- tent with the previous result on large numbers of Japanese females (3). This consistency suggests that the hair samples of this study are representative ones. In Figure 5, the el- lipticity of a cross section, defi ned by the ratio of the major to the minor axis, is plotted against age, showing that it has no dependence on age. The average value 1.28 indicates that the hairs are relatively round, compared to values for Caucasian and Ethiopian hair (1,14). As shown in Figures 4 and 5, the age dependence of both hair diameter and ellipticity does not correlate with the decrease in hair luster with age, meaning that the luster reduction with age does not arise from the changes in fi ber diameter or ellipticity. Figure 6 displays examples of magnifi ed photographs of the hair of three panelists and several hair fi bers randomly sampled from them. For the youngest (25 years old) panelist (a), the appearance looks smooth and the bright luster belt around the top of the head is Figure 4. Age dependence of minor axis of a hair cross section measured at 20°C and 65% relative humid- ity for 132 randomly selected Japanese females. Each point is the average of all the fi bers sampled from a 7-mm × 7-mm area of the top of the head for each panelist except for fi bers less than 5 cm in length. Solid curve: quadratic approximation. Filled squares with an error bar: mean ± SD of each decennial age group. Figure 5. Age dependence of the ellipticity of a hair cross section (major axis/minor axis). The panelists and measured hairs are the same as those in Figure 4. Solid line: result of linearization. Filled squares with an error bar: mean ± SD of each decennial age group.

EFFECT OF AGE ON HAIR PROPERTIES 643 clear, but for the oldest (57 years old) panelist (e), the appearance looks dull and the luster belt is diffuse. The hair fi bers of the youngest (b) are relatively straight, but the irregular curled shape increases with age (d,f). This shape difference is likely the cause of the un- even alignment of hair fi bers that leads to the decrease in the luster of hair. To clarify the actual change in hair shape with aging, measurements of the curl radius were performed on the hair fi bers sampled from the 230 panelists. Figure 7 shows the age dependence of the mean curvature (reciprocal of the curl radius) for each panelist. It is seen that the hair curliness increases with age and that the slope of the line is statistically signifi cant (p-value = 9.9 × 10−12). The index of determination r2 is 0.185, indicating that 19% of the variation in mean curvature is explained by variation with age. INTERNAL STRUCTURE OF HAIR In order to understand the cause of this irregular curl shape of hair fi bers with aging, small-angle X-ray scattering measurements were performed. By moving the target posi- tion of the X-ray microbeam on a hair fi ber from the convex side to the concave side of the curl, it was possible to detect microscopic unevenness of the nanostructure in the Figure 6. Examples of hair appearance (sensory luster) and hair fi ber shape for three panelists. Parts a, c, and e: magnifi ed back-shot images of hair appearance. Parts b, d, and f: hair fi bers randomly selected from each panelist. The ages of the panelists are 25 for parts a and b, 40 for parts c and d, and 57 for parts e and f. Figure 7. Age dependence of the mean curvature of hair fi bers for the same panelists an in Figure 3. These points are the averages of ten fi bers randomly selected from the top of the head. Solid line: result of lineariza- tion. Filled squares with an error bar: mean ± SD of each decennial age group.