SWELLING OF EPIDERMAL MEMBRANE 25 z 14_ 1.12 1.10 1.08 1.06 1.04 1.02 1.00 ß EPIDERMAL MEMBRANE (CW LENGTH) T - EACH POINT IS AN AVERAGE OF I0 REPLICAS ] _ o HAIR LENGTH / I I I I I I I I I I I0 20 30 40 50 60 70 80 gO I00 % RH Figure 4. Hair length and EM-CW length vs. RH. 2.00 1.75 1.50 1.25 1.00 ß EPIDERMAL MEMBRANE (THICKNESS) - EACH POINT IS AN AVERAGE OF I0 REPLICAS / • HAIR DIAMETER 0 I0 20 30 40 50 60 70 80 90 I00 %R.H. Figure 5. EM thickness and hair diameter.

26 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 2.00 1.75 1.50 1.25 1.00 ß EPIDERMAL MEMBRANE (THICKNESS) o EPIDERMAL MEMBRANE (CW LENGTH) EACH POINT IS AN AVERAGE OF I0 REPLICAS HAIR DIAMETER FROM STAM et. al. 0 I0 20 30 40 50 60 70 80 90 I00 % RH Figure 6. Swelling of hair and epidermal membrane. length of human EM, and furthermore the differences in these two substrates are greatest at the highest relative humidities (11). Several authors have demonstrated the effects of RH or corneum moisture content on its elasticity (12, 13, 14). Wolfram (15) has compared the effects of RH on the extensibility of corneum and hair, and Middleton (16) has demonstrated the effects of temperature on corneum extensibility. In addition, skin dimensional changes com- pound this situation because the innermost layers of the corneum are exposed to approximately 86% RH (17) and the outermost layers to the external environment. Thus, when the skin is subjected to a change from high to low RH, the horny layer tends to shrink (See Table II), but this shrinkage is not uniform throughout its depth, therefore shearing stresses are produced. So, under conditions of temperature and humidity extremes, the horny layer develops a Table II Changes in Skin Dimensions by RH from 86% ARH % Area Change* % Thickness Change 86 to 20 --6.9 -- 16.4 86 to 30 --6.1 -- 15.8 86 to 40 -- 5.5 -- 15.6 86 to 50 --4.7 -- 14.8 86 to 60 --3.8 -- 12.6 86 to 70 --2.8 --9.5 *Obtained by Squaring % A CWLength