Long-term and short-term tolerable work-time in a hot environment: the limit values verification

A group of four efficient mine rescuers at the age of 25-35 years were exposed to a load at a cyclo-ergometer (stage A and B) and a hand ergometer (stage E) in a climate chamber. The total period of work of 120 min was divieded into four work intervals, 30 min each. There were 5-min breaks between the individual intervals. The load at the ergometer was selected in the range of 25-150 W, Tg = 20-40 C, rh = 40-80% and va = 0.2-1.5 m.s-1. The thermal resistance of the working suit was 0.65 clo in the stage A, 1.07 clo in the stage B and 0.81 clo in the stage E. A total of 200 experiments with 50 combinations of the work and climate load were made. Heart rate, oxygen consumption, carbon dioxide production, body temperature, skin temperature, water loss by sweating and perspiration, dry and web bulb air temperature, air velocity and globe temperature were measured during the experiments. The expected production of sweat (SR) and the amount of accumulated heat in the body (Qmax) were calculated for each combination of the work-climate conditions by a computing programme ISO 7933:1989 as well as by our own programme.A good agreement was reached between the measured and predicted SR values, calculated by the ISO programme ( r = 0.871) as well as between the values calculated by the two programmes, respectively ( r = 0.985). The experimental results have shown a good agreement between the predicted and actually measured values of temperature of the body core as an index of short-term tolerable climate load. The values of short-term tolerable time of work calculated at the level of the amount of accumulated heat in the body of 50 W.h.m-2 resulted in the increase of body core temperature by 0.8-1.0 K. The values of heart rate mostly did not exceed 140 min-1, reaching in exceptional (three) cases slightly values above 150 min-1. The authors recommend to limit long-term workheat (climatic) load during a higher metabolic rate ( N > 80 W.m-2 include basal metabolic rate) or acclimatized males and females by the sweat rate SR = 270 g.h-1.m-2, of non-acclimatized persons SR = 206 g.h-1.m-2. The limit for low metabolic rates * M 80 W.m-2) for non-acclimatized and acclimatized persons is proposed for long-term tolerable load SR = 147 g.h-1.m-2. Short-term tolerable load by heat storage within organism for all categories is proposed Qmax= 50 W.h.m-2.