J. Soc. Cosmet. Chem. 24 239-243 (1973) • 1973 Society of Cosmetic Chemists of Great Britain The influence of temperature and humidity on stratum corneum and its relation to skin chapping J. D. MIDDLETON and B. M. ALLEN* Presented on the 11th April 1972 in Oxford, at the Symposium on' Skin--Environmental responses and protection', organized by the Society of Cosmetic Chemists of Great Britain. Synopsis The susceptibility of the STRATUM CORNEUM to CHAPPING depends upon its mechanical properties such as extensibility. It has previously been shown that at constant temperature, extensibility of isolated corneum is related to RELATIVE HUMIDITY and that at constant relative humidity, extensibility increases when the temperature is raised above 25øC. In this report extensibility of isolated animal corneum has been determined at 5øC where skin chapping occurs more frequently. EXTENSIBILITY was found to be lower at reduced tem- peratures although the corneum water content did not decrease. It was concluded that skin chapping is the result of a low corneum TEMPERATURE and WATER CONTENT and that greater flaking and chapping of the surface corneum is a result of a gradient of temperature or water content across the corneum in a cold or dry ENVIRONMENT. INTRODUCTION The function of the stratum corneum is to protect the underlying tissues from desiccation and from chemical and physical insult. To fulfil these functions it must form a continuous membrane with a high degree of flexibility and elasticity to allow for the movements of the skin. If flexibility and elasticity are reduced the corneum tends to crack and flake. This con- dition is commonly seen, particularly in winter weather, and is often re- ferred to as skin chapping. * Environmental Safety Division, Unilever Research Laboratory Colworth/Welwyn, Colworth House, Shambrook, Bedford. 239

240 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Several studies have investigated the factors which control the flexibility and elasticity of the corneum. Blank (1) was the first to observe subjectively that water was an important factor in the flexibility of human callus and that the water content of isolated callus depended upon the relative humi- dity (rh) of the atmosphere. The dependence of water content of isolated corneum on rh was later confirmed by Singer and ¾inson (2) and by Middleton (3) using animal corneum. The dependence of the elastic properties of corneum on rh was demon- strated by Middleton (3), who showed that extensibility of isolated animal corneum increased with increasing rh. Wildnauer, Bothwell and Douglass (4), working with isolated human corneum, showed that elongation of stratum corneum strips at fracture increased with increasing rh. The breaking strength decreased with increasing rh, but the work required to fracture a strip increased with increasing humidity. The dependence of corneum water content and elastic properties on humidity suggests that the incidence of skin chapping should be related to humidity. Gaul and Underwood (5) showed that the incidence of chapping was greater at low absolute humidity (dew point) and was not related to rh. However, Singer and Vinson (2) showed that corneum water content was related to rh and not to absolute humidity. Middleton (6) pointed out that in Gaul and Underwood's observations (5) low absolute humidity only occurred at low temperature and showed that, at temperatures greater than 25øC, corneum extensibility increased with temperature under conditions where water content did not increase. It was concluded that corneum ex- tensibility, and hence the incidence of chapping, was dependent upon both rh and temperature. Our previous measurements on the effect of temperature (6) were restricted to temperatures greater than ambient and in this paper we report the effect of lower temperatures in the region where skin chapping normally occurs. EXPERIMENTAL Stratum corneum from the rear footpads of guinea-pigs was used. The method of separation has been reported previously (7). From each piece of footpad corneum a strip measuring 0.6 by 1.5 cm was cut with a stainless steel punch. Extensibility was measured on an Instron Tensile Tester. Details of the technique have been described previously (3). A strip of corneum was clamped between two pairs of jaws and stretched at a con- stant rate of extension. The load is measured on a tension cell and the