The Effects of Aging on Hair–More Than Just Amount 472 these mechanical properties should occur near the late teens. Then with additional increase in age, these mechanical properties of hair fibers should decrease. Torsional rigidity will be somewhat different, as Persaud and Kamath30 have shown that the cuticle in the outer fiber layers plays a more significant role in torsional rigidity than in stretching behavior. Thus, Persaud and Kamath have shown that the shear modulus decreases with hair fiber diameter because of the higher ratio of cuticle to cortex with decreasing fiber diameter. Therefore torsional rigidity will decrease for the scalp hair of Caucasian and Asian women until near age 40 and for men until the late teens then torsional stiffness will increase with advancing age. Hair breakage is a multifactorial phenomenon involving bending, stretching and torsion deformations and includes: • Tangle formation with hair fibers looped over other hairs with severe bending deformations as shown by Brown and Swift31 • Knots that form more in hair with high curvature and are easily fractured32 • Treatments and weathering: Chemical damage increases breakage and conditioners decrease breakage33-38 • Relative humidity (RH) or water content of the hair: Highly coiled hair breaks more by dry state grooming, while straight to wavy hair provides more short segment breaks ( 2.5 cm) when dry, but more long segment breaks when wet38, 39 • Impact breakage or pulling a comb or brush through a tangle with breakage40 • Physical damage or wear by abrasion from specific grooming devices such as combs, picks or brushes and to some extent a fatiguing action.38, 40-42 Accumulated data from conditioned versus shampooed hair, bleached versus unbleached hair, and comb stroke length comparing broken hairs versus combing loads shows that hair breakage increases with combing forces.38, 43 Spearman’s non-parametric correlation test of these data provides a significant correlation for