Chapter 14 487 Male Hair Diameter vs. Age • In contrast to females, existing data on hair diameter versus age for male subjects demonstrate that hair diameter increases to a maximum in the late teenage years and then decreases relatively rapidly with increasing age. However, additional larger studies on hair diameter versus age for males are required to determine the exact age range for maximum diameter. Manifestation of Hair Diameter Changes on Consumer Hair Properties/Perception • Hair diameter has been shown to be an important contributor to scalp coverage, therefore changes in hair fiber diameter with age will have a major impact on the perception of hair thinning. • Changes in hair fiber diameter with age will change tensile, bending and torsional resistance of hairs. The increase in fiber stiffness with the diameter increase up to age ~40s for females will tend to make the hair easier to comb, whereas the decrease in diameter by itself with advancing age would increase combing forces and therefore breakage. Knowledge Gaps for Hair Diameter vs. Age • The impact of scalp site and geo-ethnic differences on hair diameter changes versus age for both females and males remain key knowledge gaps that need to be addressed. References 1. TB Fitzpatrick and G Szabo, Part III: General considerations of skin pigmentation: The melanocyte: Cytology and cytochemistry, J. Invest. Dermatol, 32, 197-209 (1959). 2. DJ Tobin, R Paus, Graying: Gerontobiology of the hair follicle pigmentary unit, Experimental Gerontol, 36, 29-54 (2001). 3. Kukita, Changes in tyrosinase activity during melanocyte proliferation in the hair growth cycle, J. Invest. Dermatol. 28, 273-274 (1957). 4. Robbins, Chemical and Physical Behavior of Human Hair, pp 181-188, Springer-Verlag, Berlin, Heidelberg, and New York (2002). 5. DJ Tobin et al, -Endorphin as a regulator of human hair follicle melanocyte biology, J. Invest. Dermatol, 123, 184-195 (2004).