Hair Breakage 266 A seemingly logical explanation for the extreme effect of relative humidity lies with the plasticization it imparts to the hair structure. That is, higher moisture content lowers the modulus of hair, which subsequently raises the deformation that results from application of a given force. By means of illustration, Table 3 shows modulus data as a function of relative humidity for Caucasian hair obtained by conventional stress-strain testing. Thus, it becomes possible to calculate the deformation that occurs from application of a given stress (in this instance 0.05 x 109 Pa) as a function of the relative humidity. As such, a common repeating stress results in larger sample deformations as the relative humidity rises and it appears instinctive that repeated higher deformations would lead to faster failure. Table 3. Young’s modulus data for Caucasian hair as a function of RH as obtained by stress-strain experiments Relative Humidity Young’s Modulus Deformation under 0.05 x 109 Pa stress 20% 3.83 x 109 Pa 1.28% 60% 3.14 x 109 Pa 1.56% 90% 2.39 x 109 Pa 2.05% Figure 11. Average cycles-to-fail for Caucasian hair as a function of fiber dimension, repeating load and RH