260 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS (15,16). A lowered refractive index should result in decreased scatter and, consequently, deeper penetration of radiation and higher apparent absorbance. Humidity-induced hydration, however, does not appear to cause a significant increase in the path length of near-infrared radiation into the skin. Under high humidity, the increase in free water content apparently gives rise to swelling of the SC, while the decrease in bulk water suggests that less radiation consequently penetrates below the SC. Thus, there is no major change apparent in the net depth of penetration of radiation or significant change in the refractive index in this region with hydration. Therefore, the humidity results provide evidence that NIR reflectance is a superficial measurement at 1879 nm since free water is presumed to be restricted to the SC. On the other hand, the presence of bulk water suggests that some portion of the reflected radiation has penetrated into the epidermis. The depth into the epidermis is likely to be small otherwise the bulk water absorbance would overwhelm the spectrum. In contrast to the humidity effects, certain products can affect scattering properties by smoothing the skin surface. Normal skin has been shown to have fewer and shallower crevices and an absence of flaky, uplifted scales compared to dry skin (17,18). The formation of a smoother surface should result in less scattering of radiation, as noted for the three formulations in Figure 10. Because all three show decreased scatter without increased free water, the mechanism by which skin is made smoother may not necessarily be hydration. In fact, it appears that no hydration per se occurs with product application in the cases examined here. The decrease in free water combined with the increase in bulk water suggests that the SC is becoming thinner, opposite to the plumping effect observed with increasing humidity. The thinning may be due to compacting of the tissue or to removal of the driest, outer portion of the SC. Thus, maintenance of at least moderate humidities and application of moisturizers may both be beneficial to skin, but through different mechanisms. Increased humidity hydrates the skin, while moisturizers of the type applied in this study appear to smooth skin. NIR reflectance thus appears to show potential as a method for evaluating both the smoothness and water content of skin. Further work to validate the treatment data with regard to separating the effects of scattering and concentration needs to be done. NIR may also offer a means for measuring SC thickness on free water/bulk water intensities. SUMMARY We have introduced NIR reflectance as a method to directly determine changes in free, bulk, and protein-bound water and to assess scattering effects in skin in the evaluation of skin care products. In addition, the effect of humidity on different types of water in skin can be examined. From our initial studies, it appears that increasing ambient humidity (up to 50% RH) is associated with increased levels of free water in the skin, while the products we used act primarily to smooth the skin surface, observed as a decrease in the scattering of radiation. Our results further indicate that although the water we see is primarily in the stratum corneum, some of the reflected radiation comes from the epidermis.

MOISTURE MEASUREMENT BY NIR SPECTROSCOPY 261 ACKNOWLEDGMENTS The author wishes to thank P. L. Walling for her initial work on NIR reflectance of skin and her continued contributions to this work, and R. Samaritano for her help in collecting the spectra. REFERENCES (1) A.M. Kligman, Regression method for assessing the efficacy of moisturizers, Cosmet. Toiletr., 93, 27-35 (1978). (2) M. Obata and H. Tagami, A rapid in vitro test to assess skin moisturizers,J. Soc. Cosmet. Chem., 41, 235-241 (1990). (3) R. O. Potts, Stratum corneum hydration: Experimental techniques and interpretation of results, J. Soc. Cosmet. Chem., 37, 9-33 (1986). (4) J. R. Hansen and W. Yellin, "NMR and Infrared Spectroscopic Studies of Stratum Corneum Hy- dration," in Water Structure at the Water-Polymer Interface, H. H. G. Jellinek, Ed. (Plenum Press, New York, 1972), pp. 19-28. (5) W. Gehring, M. Gehse, V. Zimmerman, and M. Gloor, Effects ofpH changes in a specific detergent multicomponent emulsion on the water content of stratum corneum, J. Soc. Cosmet. Chem., 42, 327-333 (1991). (6) R. O. Potts, D. B. Guzek, R. R. Harris, and J. E. McKie, A non-invasive, in vivo technique to quantitatively measure water concentration of the stratum corneum using attenuated total reflectance infrared spectroscopy, Arch. Dermatol. Res., 277, 489-495 (1985). (7) P. L. Walling and J. M. Dabney, Moisture in skin by near-infrared reflectance spectroscopy, J. Soco Cosmet. Chem., 40, 151-171 (1989). (8) P. Geladi, D. MacDougall, and H. Martens, Linearization and scatter correction for near-infrared reflectance spectra of meat, Appl. Spec., 39, 491-500 (1985). (9) K. A. Martin, Recent advances in near-infrared spectroscopy, Appl. Spectrosc. Rev. 27, 325-383 (1992). (10) J. D. Hardy, H. T. Hammel, and D. Murgatroyd, Spectral transmittance and reflectance of excised human skin, J. Appl. Physiol., 9, 257-264 (1956). (11) R. R. Anderson, J. Hu, and J. A. Parrish, "Optical Radiation Transfer in the Human Skin and Applications in In Vivo Remittance Spectroscopy," in Bioengineering and the Skin, R. Marks and P. A. Payne, Eds. (MTP Press, Lancaster, England, 1981) pp. 253-265. (12) B. G. Osborne and T. Fearn, Near Infrared Spectroscopy in Food Analysis (Longman Scientific and Technology, New York, 1986), pp. 49-51. (13) W. A. Luck, "Hydrogen Bonds in Liquid Water," in The Hydrogen Bond, Vol. III, P. Shuster, G. Zundel, and C. Sandorty, Eds. (Elsevier-North Holland, Amsterdam, 1976), pp. 1369-1423. (14) R. J. Scheuplein, A survey of some fundamental aspects of the absorption and reflection of light by tissue, J. Soc. Cosmet. Chem., 15, 111-122 (1964). (15) J. L. Solan and K. Laden, Factors affecting the penetration of light through stratum corneum, J. Soc. Cosmet. Chem., 28, 125-137 (1977). (16) P. T. Pugliese and A. J. Milligan, "Ellipsometric Measurement of Skin Refractive Index In Vivo," in Bioengineering and the Skin, R. Marks and P. A. Payne, Eds. (MTP Press, Lancaster, England, 1981) pp. 291-302. (17) A. J. Quattrone and K. Laden, Physical techniques for assessing skin moisturization, J. Soc. Cosmet. Chem., 27, 607-623 (1976). (18) T. M. Kajs and V. Garstein, Review of the instrumental assessment of skin: Effects of cleansing products, J. Soc. Cosmet. Chem., 42, 249-271 (1991).