206 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 1.040 - 1.030 • 1.020 o o ' 1.010 "• 1.000- (.) z ,• 0.990- o c• 0.980- o. 970 0.960 r = - 0.409 n =307 p 0.001 0.950 I I , , , • a 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 SCALE SCORE 1.07 - 1.05 - 1.03 - 1.01 - 0.99 0.97 N 1" t NNNN N N N r = - 0.344 n = 307 p 0.01 0,95 I I I • I I I 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 b ROUGHNESS SCORE Figure 7. a: Correlation between the NIR absorbance and the "scale" score. b: Correlation between the NIR absorbance and the "roughness" score.

NEAR-INFRARED SPECTROSCOPY 207 9O 86 82 78 74 7O 66 \\ \\ \ \ \ \ \ \ \ N \\ \ \ \ r = - 0.439 n = 307 p = : 0.001 62 , i i i i i i i 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 GLOBAL SCORE Figure 8. Correlation between the electrical conductance in voltage and the global score. The statistical parameters correspond to a linear correlation obtained with the whole data. The correct classification of products in terms of their efficacy on dry skin was therefore given by the NIR absorption method and the clinical scoring technique. As the statis- tical analysis showed a significant difference between NIR results for product B and products A, C, and E, this is probably the most sensitive technique for distinguishing between different cosmetic preparations. The conclusions of our study concern both the nature of dry skin and its assessment (and consequently the efficacy of topical agents). The results clearly show that dry skin is indeed dehydrated, and confirm hypotheses based on indirect methods showing that the stratum corneum of extremely dry skin is almost two times less elastic than that of normal skin (2). This fundamental observation should now form the basis for studies of the physiological phenomena that give rise to abnormal keratinization, and should also lead to the development of new and more effective moisturizers. The results of the NIR absorption method, which can be applied in vivo to.all skin sites, were strongly correlated with the clinical scores the intrinsic property'it measures can be directly interpreted in terms of water content, something that was not previously possible. Indeed, although the electrical conductance method is simple and informative, the present study clearly shows its limitations. We have often insisted on the complementary nature of the various methods used to study the nature of dry skin and the efficacy of cosmetic products (10) the results of this work confirm our view but illustrate the exceptional value of the NIR absorption method for measuring a parameter fundamental to cosmetology, i.e., skin water con- tent.