340 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Procedures Skin impedance is always determined on the distal face of the forearm. Test-subjects were volunteers from the laboratory personnel. At least 15 min before the determinations, test sites are standardized: a swab of cotton-wool soaked with physiological saline is passed rapidly over the test areas and the skin dabbed dry directly afterwards. To determine a-relaxation parameters, measurements of Z and 0 are taken at fourteen different frequencies in the range 5 Hz-1 kHz. Starting at the low-frequency end of the relaxation, measurements are made at the intermediate frequencies up to 1 kHz, then back to check the values of Z and 0 at the two lowest measuring frequencies. When these values are different from the initial ones, the subject is rejected. Each determination takes about 20 min. Relaxation parameters are deduced from Z and 0 values obtained at each measuring frequency. After subtracting the impedance of the electrodes, the parallel components of the equivalent circuit are calculated, i.e. the conductance G = cos O/Z and the capacitance C =--sin O/coZ. From these data, the Cole and Cole locus in the complex permittivity plane is plotted and a fitting procedure using an IBM 360-40 computer yields the values for: Go, static conductance Co, static capacitance Cc•, capacitance after relaxation fc, characteristic relaxation frequency ß , relaxation time- 1/2•fc- 1/co a, distribution parameter of relaxation time Az, dielectric decrement. Low-frequency determinations are quickly made since they are effected at a single frequency (25 Hz) to yield Z (25 Hz). In the present paper, the development of suitable electrodes is described. Main a-relaxation data relating to human skin are given and it is shown how dehydrafion-hydration experiments affect electrical skin parameters. Finally, attempts are made toward explaining the dielectfical skin behaviour in the a-relaxation frequency region by means of a theoretical model. EXPERIMENTAL The contribution of the stratum corneum to Z was established by deter- ruination of a-relaxation parameters before and after repeated stripping (10-20 times) with adhesive tape (eight subjects).

JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 2. Skin impedance measurement. Fach•g page 340