Chapter 14 485 Curvature • Hair fiber curvature increases with age for Japanese females, which had a negative impact on hair luster. • An increase in hair fiber curvature with age would be predicted to result in the appearance of frizziness due to a lack of synchronization in the curvature of neighboring hair fibers. Ellipticity • Existing data supports an impact of age on hair ellipticity in early childhood however, larger studies after 10 years of age have shown no effect of age on hair ellipticity for Caucasian and Asian females. Stretching, bending and torsional properties of hair fibers • Combing forces have been shown to increase with increasing hair fiber curvature, and to decrease with increasing fiber diameter. Therefore, we would expect that combing forces would increase at advanced age as curvature increases and fiber diameter decreases, leading to increased breakage. However decreasing hair density with age must also be considered, which would offset these combing force increases, the extent of which must be determined. • Hair tangle formation is a key cause of breakage. Smaller diameter hairs break more readily than larger diameter hairs because they are less stiff and they bend and tangle more readily. Similarly, hair with higher curvature forms more complex tangles than lower curvature hair and thus breaks more readily. Therefore, for many female adults hair breakage should increase as these hair parameters change with age. Lipids • Both hair lipid levels and hair lipid composition change with age. Corresponding changes in sebum production with age suggest a role for sebaceous gland activity in age-related lipid changes in hair.