56 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS i i i I i i i i ! i \ i - i ! i o I 1 p.m. a.m.p.m.a.m. p.m.a.m. p.m.a.m.p.m. Mon. Tues. Wed. Thurs. gri. Figure 9. Moisture uptakes of permeable and impermeable leathers in the modified SATRA 'PA' test during 5 consecutive days. Moisture loss took place overnight at 65 % rh, 20 DC. - , Permeable leather ...... , impermeable PVC-coated leather. (ii) If sweat accumulates within the hose and shoe some is re-absorbed by the skin and in extreme cases it becomes very soft and easily damaged by abrasion. The normal moisture content of the skin is about 72•o (14) but it can rise substantially above this level when certain types of impermeable shoe materials are worn. There is no published data for the physico- mechanical properties of water swollen skin 'in vivo' although tensile measurements on normal skin have been reviewed by Wright (15). (iii) subjective observation suggests that the coefficients of friction of foot skin and shoe materials increase as their moisture content rises. Thus

HEALTH AND HYGIENE OF FOOT SKIN 57 (a) 3.0 2.5 2.0 1.5 1.0 0,5 0 Side leather Corfom //• Calf leather / •//Clarino Parrair I I I I 0.5 1.0 1,5 2,0 1.5- (b) 0 I l 0 0,5 1,0 1,5 2,0 0.5 Distension (mm) Corforn Side leather Calf leather Parrair C larino Figure 10. (a) Load-distension properties of five upper materials after conditioning at 65 % rh, 20øC. (b) Area load-distension properties of the five upper materials after 7 h 'PA' conditioning. the combination of softer and more easily damaged skin and localized friction and pressure effects can readily lead to skin damage and infection. Although permeable leather uppers maintain the foot skin at a rela- tively normal moisture content this is not to suggest that leather shoes cannot cause skin damage and many of us will have had personal experience to the contrary. The reason lies in the fact that leathers display a very wide range of physicomechanical properties and are affected to different degrees by absorbed moisture. Generally low-price leather can be harsher and less affected by moisture absorption than more costly leather and this is no doubt a contributory factor in some instances. It must not be overlooked however, that shoe and last design and other shoe manufacturing aspects also play a very important part in this. THE INFLUENCE OF SWEAT ACCUMULATION ON FOOT HEALTH AND HYGIENE The mechanical aspects of this have been considered briefly in the previous section but one point might be made in connection with sweat rates