The thermophysiology of uncompensable heat stress - Physiological manipulations and individual characteristics

In many athletic and occupational settings, the wearing of protective cloth ing in warm or hot environments creates conditions of uncompensable heat st ress where the body is unable to maintain a thermal steady state. Therefore , special precautions must be taken to minimise the threat of thermal injur y. Assuming that manipulations known to reduce thermoregulatory strain duri ng compensable heat stress would be equally effective in an uncompensable h eat stress environment is not valid, In this review, we discuss the impact of hydration status, aerobic fitness, endurance training, heat acclimation, Sender, menstrual cycle, oral contraceptive use, body composition and circ adian rhythm on heat tolerance while wearing protective clothing in hut env ironments. The most effective countermeasure is ensuring that the individua l is adequately hydrated both before and throughout the exercise or work se ssion. In contrast, neither short term aerobic training or heat acclimation significantly improve exercise-heat tolerance during uncompensable heat st ress. While short term aerobic training is relatively ineffective, long ter m improvements in physical fitness appear to provide some degree of protect ion. Individuals with higher proportions of body fat have a lower heat tole rance because of a reduced capacity to store heat. Women not using oral con traceptives are at a thermoregulatory disadvantage during the luteal phase of the menstrual cycle. The use of oral contraceptives eliminates any diffe rences in heat tolerance throughout the menstrual cycle but tolerance is re duced during the quasi-follicular phase compared with non-users. Diurnal va riations in resting core temperature do not appear to influence tolerance t o uncompensable heat stress.