PREPRINTS OF THE 1996 ANNUAL SCIENTIFIC MEETING 251 The effect of chronological aging on hair growth and hair properties MIKLOS M. BREUER, School of Pharmacy, University of Rhode Island. INTRODUCTION Changing demographics suggests that large populations in North America, Europe and Japan will become old in the next century. Little is known about the way aging might affect the hair care/hair grooming industry. The purpose of this paper is to review the available knowledge on the effect of aging on hair. EFFECTS OF WEATHERING AND AGING Environmentally induced effects (e.g., damage owing to UV and visible light, humidity changes, pollution, etc.) are known as hair weathering, and are generally cumulative, i.e., increase with time of exposure and, therefore, they progressively increase along the length of hair fiber from root to tip. On the other hand, changes that occur in hair fibers irrespective of environmental exposure will affect the fibers in their entireties. These can be attributed to physiological changes of the human body owing to three causes: aging, nutritional deficiencies, or illnesses. The present paper reviews the published scientific literature that is pertinent to changes caused by aging. Aging induces a number of physiological changes in skin and its appendages. In general, the progression of these changes increases with chronological age. However, the rate at which individual persons age varies greatly. HAIR LOSS Both for men and women the density of hair fibers on the scalp gradually decreases with age. According to Courtois et al. (1), the rate at which hair fibers on the scalp are lost varies from individual to individual. This is due mostly to differences in the growing (anagen) phases of the hair growth cycles. For a given individual, the length of the subsequent anagen phases diminishes with age. At the same time the resting (telogen) periods become longer. Therefore, as an individual ages, increasingly fewer of his hair follicles will be active at any given time and will produce shorter length hair fibers. Whereas this process generally occurs with advancing age, for some males, the process can be greatly accelerated, leading to premature scalp hair loss (i.e., male pattern baldness syndrome (alopecia)). Simultaneously with decrease in the density of active hair follicles on the scalp, the diameter of the hair fibers produced also diminishes with age, leading to an increased perception of scarce scalp hair in older persons. GROWTH RATE The actual rate of scalp hair growth has been found to be independent of age. On the

252 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS other hand, some published results also suggest that the growth rate of nails gradually diminishes with age (about 0.5%/year). CHANGES IN HAIR PROPERTIES Whereas the effect of aging on the hair cycle and growth has been fairly extensively studied, only sporadic data are available on changes that occur in hair fiber properties due to aging. The few available published papers suggest that aging does not signifi- cantly affect the morphology of hair tissues, but that it influences the mechanical properties of newly grown hair. Fibers recovered from heads of older persons seem to be less strong and less elastic than those that grow on young persons' scalps. No data are available on changes of chemical composition of hair fibers owing to aging. Conse- quently, it is difficult to explain the reasons for the observed changes in mechanical properties. More recent work has not found significant differences between the properties of grayand pigmented fibers. Anecdotal evidence also suggests that the properties of body hair (pubic, chest, facial axillary hair) change with age, albeit in the opposite direction to those of scalp hair. GRAYING OF HAIR Hair graying is one of the major indicators of aging and as such has been extensively studied. The number of functioning melanocytes feeding melanin granules into the hair shaft gradually declines with age, bringing about the graying process. The rate of graying has been found to vary greatly from individual to individual and is probably determined by genetic factors. The extent of graying and that of alopecia appear to be weakly correlated, suggesting that both these phenomena are indicators for an under- lying general aging process. HAIR GRAYING, ALOPECIA, AND DISEASES A number of diseases lead to acute alopecia and also, possibly, premature graying. Danish workers found correlations between hair graying, male alopecia, and the inci- dence of coronary heart disease (2). According to these researchers, the relationships suggest that all three quantities are indicators of a general biological aging process. CONCLUSIONS Hair graying and hair loss are good indicators of the human aging process. Sporadic data suggest that the hair properties of older individuals differ from those of young and middle-aged persons. In view of the anticipated future increase in the proportion of older people among consumers that are expected to purchase hair grooming products, a better understanding of the properties of the hair of older persons would appear to be desirable. REFERENCES (1) M. Courtois, G. Loussouarn, C. Hourseau, and J. Grollier, Br. J. Dermatol., 132, 86 (1995). (2) P. Schnohr, P. Lange, J. Nyboe, M. Appelyard, and G. Jensen, Am. HeartJ., 130, 1003 (1995).