Evaluating the performance of antiperspirants 417 Test products are applied in a randomised pattern on six sites, constant throughout the test week, and four control sites situated as indicated in Fig. 2. 0.1 ml of test product is applied from a syringe to an area 3 cm square (so that the area treated overlaps the collection area) and is allowed to dry for 1 h in a room at 23øC. At the end of this period a dried 2 cm square pad made from surgical pad gauze* backed by a 5 cm square sheet of occlusive, adhesive taper is fixed over the collection site. Arrangement of collechng system •k,• Spine lectoutsitescutbetweencmcm2_cm sit 2_.5 Oo, 2_ x 7'5 Figure 2. Gravimetric sweat collection on the back Position of control sites (C) Group ] Group 2 The subjects then enter the hot-room (38'5øC 35 % RH) for a period of 45 min. At the end of ths period the patches are removed and weighed. They are weighed 24 h later after drying in a desiccator. Under these conditions control site patches typically collect up to 200 mg of sweat. This compares with up to 2,500 mg over 40 rain over the whole axillary vault. The efficacy of products can be numerically higher from back screening tests than axillary tests. Nevertheless the back screening test has the advantage that six products can be compared with each other directly and it is unlikely that a usable formulation will be rejected as a result of a back screening test. HYGROMETRY As mentioned earlier the cooling effect of sweating is brought about by the evaporation of water at or below the skin surface, thus as ambient air flows over the skin its moisture content (relative humidity) is increased. The product of (the increased water content) x (the air flow rate) is the rate at which sweat is evaporating at the skin surface. Provided that sweat droplets are not forming on the surface the rate of evaporation equals the rate at which sweat is emerging from the sweat ducts. * Surgipad, Johnson & Johnson Ltd, Slough, England. •- Sleek tape, S & N Southalls Ltd, Welwyn Garden City England

418 W. B. Davis and A.M. Rees-Jones In order to measure the rate of sweating, cylindrical cells (Fig. 3) are fixed to treated and control sites on the backs of subjects. 'Ambient' air (38.5øC 35•o RH) is drawn from the warm room and pumped into the cells at the rate of 150 ml per min through twelve 1'5 mm diameter holes situated concentrically inside the cells. The cells are held in dose contact with the skin by 3 cm wide strips of elastic thus preventing gross leakage. The outgoing air from each cell is directed through a humidity sensor*. The changes in capacitance of the sensor (related to humidity changes) are electronically processed and amplified before being recorded directlye' as percentage relative humidity. The air flow rate (150 ml/min) was found to be sufficiently fast to prevent the accumulation of liquid sweat on the test area. Figure 3. Hygrometric sweat measurement cell. The perspex cell directs air onto the sweating skin surface via a concentric ring of holes. The outgoing air has an elevated water content as a result of sweat evaporation. If any leakage occurs at the cell/skin boundary the air escaping will have picked up as much moisture as monitored outgoing air. Thus the product of the humidity rise and the ingoing flow rate are the relevant measurements. The outgoing flow rate is used to check for gross leakage. On entering the hot-room subjects were found to differ in their response to the thermal stress applied mainly in that the time required for sweating to increase to a consistent rate ranged from 10 to 30 min. The plateau values themselves varied from person to person and from day to day for each person. THERMOGRAPHIC MEASUREMENTS A sweat rate equal to 50 mg/min from the axillary vault draws 120 J/min from the skin surface to evaporate the sweat. When the room temperature is at or near skin temperature radiation causes no heat loss or gain, thus all the heat generated by the body should be lost by evaporation from the skin and lungs to maintain equilibrium. Under these con- ditions the skin surface is the coldest part of the environment. Localised cooling of the skin by evaporation contributes to thermal regulation, therefore it is reasonable to assume that there is less cooling of the skin over areas where an effective antiperspirant has been applied, i.e. the skin temperature is higher. Thermo- graphy, is in effect, the process of recording variations in intensity of long wavelength emissions from a surface in a mode of action similar to that of a visible wavelength television system. Hot areas emit more energy in the sensitive range of the instrument than cold areas so are thus displayed on a television screen as brighter areas in mono- chrome systems, or as a particular hue in colour systems displaying temperature vari- ations over the skin surface as a map (Figs. 4-7). * HP4 humidity & temperature probe, Lee Dickens Ltd, Kettering, England. •' Speedomax W Multipoint Recorder, Leeds & Northrup Ltd, Birmingham, England.