EVALUATION OF METHODS FOR MEASUREMENT OF ANTIPERSPIRANCY 253 Number of subjects showing a significant effect (10 % level) Product 1 Product 2 Decreased sweating 4 5 Increased sweating 2 1 No effect 7 7 (5) The results can be shown as the number of times each product is significantly better than the other. This probably does not exhaust all the possible ways of comparing the results and it is therefore obvious that the interpretation of these results can vary. It is sometimes more difficult to interpret the results than obtain them! Continuously recording methods The most sensitive methods, so far devised, to measure the onset and progress of sweating are those which fall into this last group. They can be divided into those methods which measure the changes in the electrical properties of the skin and those which measure the amount of water secreted by the sweat glands. Measurements of electrical skin resistance (ESR) have been used to quantify perspiration (32-36). Although extremely sensitive there is some doubt as to their validity. It has been suggested by Lloyd (37) that the ESR is not directly proportional to the volume of sweat secreted, but is more dependent on the number of sweat ducts filled and the ionic composition of the sweat. Further doubt on the usefulness of these methods was cast by the work of Perry, Mount and Maliner (38) who re- ported that they could show no change in the galvanic skin response after treatment with aluminium salts, which are known to decrease perspiration. The most accurate methods available are those using electronic hygro- meters. In principle all these methods (39-44) are the same. A cup is attached to the skin and the water from the enclosed area is evaporated by a constant stream of dry gas. The water content of this gas stream is monitored and the sweat rate calculated. The differences between these methods lie in the type of moisture detector used. Albert and Palmes (39) used an infra-red gas analyser, James (44) an electrolytic water analyser, whilst the others used different types of resistance and capacitance hygrometers. Resistance and capacitance hygrometers measure relative humidity, and as such are very sensitive to changes in temperature. The electrolytic water analyser, as used by James, is particularly suitable for this sort of

254 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS work. It consists of a flowmeter and a coil coated with phosphorous pent- oxide. The gas whose water content is to be measured, is passed at a con- stant rate over this coil and the moisture is absorbed by the phosphorous pentoxide. An electric current passes through this coil and is continuously measured. This current, which is proportional to the moisture absorbed by the coil, removes the water by electrolysis. This apparatus is insensible to changes in relative humidity caused by changes in temperature of the gas, providing a constant flow is maintained through the instrument. The antiperspirant evaluation procedure consists of using two such in- struments to record from two sites simultaneously (Fig. 4). This type of method is not particularly useful for recording from the axillae because as well as the objection encountered for the axillary gravimetric methods it has been shown that the axilla is not an area of even sweating (45). Because the cell used to cover the skin is not very large and only encloses a small area DRY NITROGEN MOISTURE • / MOISTU DETECTOR DETECTOR PEN RECORDER Figure 4. A diagrammatic representation of a continuously recording technique for the measurement of sweating.