532 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table X. Consumer test 3. Mean hand scores after using hand lotions Hand lotion Hand score Control 6.42 5 % lactic acid 5.36 10 Yo lactic acid 5.32 Difference required for significance (P = 0.05) 0.90 107 women completed the test. not only effective for women with severe skin dryness, but can also be expected to be of benefit in cases where there is relatively little dryness. DISCUSSION The results of the experiments on isolated animal corneum showed that increasing the water holding capacity by the addition of humectants resulted in an increased corneum extensibility. The experiments also indi- cated that the use of a humectant, lactic acid, which was adsorbed by the corneum allowed the treated corneum to be subjected to a prolonged washing without all the effect being lost. The results of the animal experi- ments predicted that lotions containing humectants, such as sodium lactate, which are not adsorbed might be less effective than lotions containing humectants such as lactic acid which are adsorbed. The results of the con- sumer tests showed that these predictions were valid and that animal cor- neum is a useful model system for studying effects in the human. The precise mechanism of action of lactic acid in reducing hand skin dryness and flaking is not clear. The results on animal corneum (Tables VI and VII) showed that corneum treated with lactic acid and then rinsed still retained an increased extensibility but there was no residual increase in water holding capacity. The explanation may be that the quantity of lactic acid adsorbed to the corneum after rinsing is too small to hold sufficient water for the gravimetric method to detect. Alternatively, the adsorbed lactic acid may itself have a direct effect on extensibility. Some separate, unpublished experiments on non-hygroscopic carboxylic acids, such as mandelic acid, indicated that these acids can increase extensibility of animal corneum without increasing water holding capacity, and that the increased extensibility survives rinsing the corneum. This suggests that lactic acid may have a direct effect on a corneum extensibility without influencing the water content.

SKIN CREAM FOR DRY AND FLAKY SKIN 533 It is extensibility and not water content which is important for the corneum in its resistance to flaking and cracking. The water content is only important in so far as it affects extensibility. Lactic acid, therefore, appears to have a dual action on the corneum. It increases extensibility by increasing the water holding capacity in the same way as conventional humectants. This effect is lost on rinsing the corneum in water, but is sufficient to result in some effect on skin dryness and flaking as shown in the consumer tests with sodium lactate lotions. The second effect, which is not shown by con- ventional humectants, is that the adsorbed lactic acid increases extensibility. In treated and unrinsed corneum, there is no evidence of an additive effect between the extra water and the adsorbed lactic acid, and the effect of the adsorbed lactic acid only becomes apparent on rinsing. This means that the effect of lactic acid is likely to be more persistent than that of sodium lactate, and should therefore result in less skin dryness and flaking. This was confirmed in the results of the second consumer test where the comparison of the sodium lactate and lactic acid lotions showed that the lactic acid lotion results in a significantly better hand skin condition. (Received: 8th March 1974) REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9) (lo) (•) Kligman, A.M. In: Montagna, W. and Lobitz, W. C. The epidermis 410 (1964) (Academic Press, New York). Middleton, J. D. The mechanism of water binding in stratum corneum. Brit. J. Dermatol. 80 437 (1968). Middleton, J. D. The effect of temperature on extensibility of isolated corneum and its relation to skin chapping. Brit. J. Dermatol. 81 717 (1969). Middleton, J. D. The influence of temperature and hun-fidity on stratum comeurn and its relation to skin chapping. J. Soc. Cosmet. Chem. 24 239 (1973). Speir, H. W. and Pascher, G. Zur analytischen und funktionellen Physiologie der Haut- oberfl•iche. Hautarzt 7 55 (1956). Jacobi, O.K. About the mechanism of moisture regulation in the horny layer of the skin. Proc. Sci. Sect. Toilet Goods Assoc. 31 22 (1959,. Blank, I. H. Factors about which influence the water content of the stratum corneum. J. Invest. Dermatol. 18 433 (1952) Middleton, J. D. The mechanism of action of surfactants on the water binding properties of isolated stratum corneum. J. Soc. Cosmet. Chem. 20 399 (1969). Acheson, D. T. In: Humidity and moisture measurement 3 521 (1965) (Reinhold, New York). Harry, R. G. In: The principles and practice of modern cosmetics 2 580 (1963) (Leonard Hill Books Ltd, London). Barker, S. B. and Summerson, W. H. The colorimetric determination of lactic acid in biological material. J. Biol. Chem. 138 535 (1941).