544 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Faster Table 11 Nail Growth Factors Slower No Effect Males Pregnancy Third digit Piano playing Onychophagia Females Lactation First digit Aging Severe cold Acute infection Pneumonia Mumps Malnutrition Decreased circulation Smoking Congestire heart failure Paralysis Sleep Left- or right-handedness Season Minor chronic illness Occupation Dietary habits Moderate emotional stress Minor surgery Height and weight Weight change Color Nail polish and remover Figure 8. Nail Growth Index slope at ten-year intervals

KERATIN REPLACEMENT AS AN AGING PARAMETER 54,5 o.q 0 0.85 0•0. z o.? s Figure 9. Yearly Nail Growth h]dex [or twenty years DISCUSSION A significant decline with age in the rate of nail growth was found in this study. Young males show faster growth rate than females this difference diminishes toward middle age and reverses itself after the seventh decade of life. This may be correlated with the "male meno- pause." The rate of growth is not noticeably affected by mild climatic changes, although severe changes in temperature have been shown to alter this rate markedly (11). Table II is a summary of various factors which are reported to alter nail growth rate (11-17). The table also lists those factors that have already been shown to have no effect. This technique of measuring the rate of nail growth can be used to monitor the potential effects of nail cosmetics and of products alleged to influence nail growth. The results of this study are in agreement with those of several previous experiments on the rate of linear nail growth. Bean (12, 13) performed a 20-year study of his own thumbnails, measuring the number of days which a scratch at the proximal edge of the nail plate took to grow out to the distal end. Converting his data into millimeter growth per week, his NGI decreased from 0.87 at age 32 to 0.82 at age 42 and to 0.73 at age 52 (Fig. 8). Figure 0 represents Bean's Nail Growth Index as measured each year from age 32 to age 52. Cross-sectional studies of linear nail growth have been performed by Hillman (17), who measured 300 individuals, and by Hamilton (16), who studied over a thousand subjects. Their average measurements•