I INFLUENCE OF AIR-PERMEABILITY OF NBC SUITS ON VAPOR RESISTANCE AND EVAPORATION OF SWEAT E.A. den Hartog, L.J.A. Wammes and G. Havenith Thermal Physiology Group, TNO Human Factors Research Institute, Soesterberg, The Netherlands %? INTRODUCTION It has been found that the suits for protection against Nuclear, Biological and Chemical Wqfare (NBC), which are currently used in the Dutch army, are not suitable for hot climates. The heat strain of the wearer is too high and the body temperature rises quickly. Recently, a new NBC suit became available. While maintaining protection, this new suit has an outer layer with an increased air - permeability (0.73 ms at 200 Pa) compared with the current NBC suit (0.37 ms at 200 Pa). This should lead to an improved ventilation of the microclimate under the suit and therefore, lead to a decrease in vapor resistance. As a conse- quence there should be an increased possibility for the wearer to evaporate sweat, leading to increased tolerance times for working in the heat. To evaluate the expected benefits, we performed 2 studies comparing the current and new NJ3C suits with respect to the vapor resistance and the total heat strain to the wearer. METHODS Ventilation measurements. The vapor resistance was determined in a series of experiments on 3 subjects by determination of the ventilation of a tracer gas (Argon) at room temperature. The method has been described extensively by Lotens and Havenith (1,2). The subjects wore a harness of polyethylene tubes that blew air, enriched with 10% Argon, under the NBC suits. A similar harness was used to suck.out air at the same rate. Both harnesses were connected to a pump that provided constant airflow. The concentrationsof Argon in the airflow to the suit (Cin), coming out of the suit (Cout), and in the surrounding air (Cair) were measured with a mass spectrometer. From these concentrations the ventila- tion under the suit was calculated by: In which V'pump is airflow generated by the pump. To calculate the ventilation only, the relative differences of the concentrations are used; therefore, it was not necessary to calibrate the output signal of the mass spectrometer. From the ven- tilation (Vent) the vapor resistance can be calculated by: DA~ -AD .60X106 (2) d = Vent 183