Heat tolerance and aging.

Although children and older adults appear more susceptible to heat stress, the mechanisms responsible for their lower tolerance are not fully understood. Many studies dealing with the effect of age on temperature regulation have concluded that an inadequate sweating response is primarily responsible for the low tolerance of children and the elderly to exercise in the heat. However, the dependence of core temperature on relative exercise intensity and sweat rate on absolute exercise intensity makes it difficult to avoid the confounding effects of exercise on thermoregulation when aerobic power (Vo2max) varies across age groups. When 38 non-acclimatized females, ages 12 to 68 years, exercised at 30-35% Vo2max in the heat, the degree of cardiovascular stability was the primary predictor of tolerance time. Age was not a significant predictor. However, it was evident that individuals at either end of the age continuum were more likely to be at risk. For children this risk was associated with the instability of an immature cardiovascular system; for older women, a marked decrement in aerobic power. Sweat rate added significantly to the prediction of tolerance time for all subjects regardless of age. Whether the decrease in responsiveness of sweating noted for some older individuals is an age related change or a reflection of their lower fitness levels is not known.