Chapter 14 479 such changes. For more complete and more speculative discussion of these effects, see the attached references.45,46,57,58 Hair from different scalp sites is an important knowledge gap: Changes in hair diameter and density with age, two key parameters involved in the perception of scalp coverage, have been studied on a limited number of scalp sites.15-24 Mirmirani and Dawson et al.24 reported a study on 44 Caucasian women and concluded that mean hair fiber diameters in the frontal site of these women was higher on pre-menopausal versus post-menopausal women, but they observed no significant difference in the occipital region. This study demonstrates the importance of determining the effects of age on hair from different scalp sites, especially during the pre-menopausal versus post-menopausal years. Since the onset of menopause on average for Caucasians and Japanese is age 51,59,60 we would expect similar effects among the different geo-racial groups. Studies on pre-teenage versus post-teenage years would also provide important information because of the large hormonal changes at puberty and the effects on hair diameter. The impact of diameter and hair density on hair coverage and its perception: Robbins and Dawson et al.16 described the effects of diameter and hair density on age and proposed a new metric “relative scalp coverage” for the perception of the amount of hair on one’s head. When considering only diameter and density, this metric is defined as a two-dimensional parameter as: the average fiber cross-sectional area multiplied by the number of hair fibers per square centimeter. Considering only diameter and density, relative scalp coverage was calculated to peak at age 35. This peak age is produced because hair diameter increases until about age 45 but density peaks in the late twenties therefore, the combination of these two important contributors to scalp coverage provides a peak for relative scalp coverage at age 35. Robbins and Dawson proposed that when other important parameters are considered for relative scalp coverage, it will provide a three-dimensional parameter involving diameter, density, fiber