INFLUENCE OF TEMPERATURE AND RH ON STRATUM CORNEUM 241 load-extension curve plotted automatically on a recorder. From the slope of the initial part of the curve extensibility can be calculated and expressed as percentage extension 100 g-• load. During extensibility measurements temperature was controlled by means of a specially constructed cabinet (LEC Refrigeration Ltd, Bognor Regis). This cabinet, with glove holes at the front, surrounded the jaws of the b•stron and had apertures at top and bottom to allow free movement of the jaw attachment to tension cell and crosshead. Relative humidity was not controlled but was measured with a wet and dry bulb thermometer with- in the chamber. Throughout each experiment temperature did not vary by more than -4- 1 øC and rh by more than + 3•o. Before measuring extensibility the corneum was preconditioned by equilibrating in a desiccator over saturated potassium bromide (81• rh at 25øC) for 6 days. It was then placed in the chamber on the Instron and left to equilibrate to the required temperature for at least 20 min before starting measurements. Water binding was measured by comparing wet and dry weights as previously described (3), after equilibrating for 6 days over saturated potas- sium bromide solution (81•o rh at 25øC) at the required temperature. Results were expressed as mg water bound 100 mg -• dry weight of corneum. The two pieces of comeurn from each guinea-pig were kept together and each piece was measured at a different temperature. In this way statistical comparisons between two temperatures could be made by analysis of variance using each animal as a statistical block. In two separate experiments a comparison between extensibility at 4-5øC and at 18-22øC was made. In the second experiment water binding was measured on the same pieces of corneum. RESULTS Table I shows the mean extensibility obtained in the two experiments together with the mean values for water binding in experiment 2. Analysis of variance showed that in both experiments extensibility was significantly less at the lower temperature with a probability of the difference arising by chance of less than 1 •o in each experiment. This difference oc- curred although the rh was higher at the lower temperature and extensi- bility is known to increase at higher rh at constant temperature (3). The measurements of water binding indicated a higher water content at lower temperatures and this difference was statistically significant (P 0.05).

242 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table I Extensibility and water binding of guinea-pig footpad corneum at different temperatures Temperature Relative Extensibility Water bound humidity (% 100 g-X (mg 100 (øC) ( • load) dry wO Experiment 1 Experiraent 2 4.8 69 4.6 - 18.9 62 6.3 - 4.0 80 0.8 - 22.0 58 1.3 - 4.0 86 - 26.1 22.0 81 - 20.6 12 replicates in experiment 1, 11 in experiment 2. This difference can be accounted for by the increase in rh over saturated potassium bromide solution with decreasing temperature (8). The decrease in extensibility at low temperature cannot be accounted for by a decrease in water content and must be due to a direct effect of tem- perature on the corneum. DISCUSSION The results show that at low temperatures corneum extensibility is reduced. From these and previous results (3, 6) it can be concluded that extensibility of isolated animal corneum depends upon both the tempera- ture and rh of the air in contact with the corneum. If this conclusion is applied to human corneum in vivo it can be predicted that these conditions will lead to skin chapping. Gaul and Underwood's observations (5) showed that chapping occurred when the air temperature was low out of doors although rh was high. However, at the same time rh indoors will be low particularly in centrally heated buildings where external air is warmed without humidifying. During a normal day, skin will therefore be exposed to the two conditions, low rh indoors and low temperature out of doors, which the experiments on isolated corneum indicate will lead to skin chapping. When living skin is exposed to low temperature or low rh there will be a gradient of temperature or water content across the corneum with the outer layers being colder or drier than the deeper layers. The result will be a lower extensibility on the surface of the corneum so that when the skin is stretched or flexed the surface layers will crack more easily, while the deeper