different age and gender, they are primarily due to differences in aerobic power or heat acclimatization.

In the 1960s, studies of temperature regulation at rest and during exercise in the heat concluded that women were less tolerant of exercise in the heat than were men (Morimoto et al., 1967; Wyndham et al., 1965); however, these investigators did not match their subjects for aerobic power or body weight-to-mass ratio. Weinman et al. (1967) were the first to suggest that gender differences could be explained by differences in physical fitness.

With regard to the effects of age on exercise-heat tolerance, it is well accepted that the aged are more susceptible to thermal injury than their younger counterparts during heat waves. This apparent heat intolerance among the aged has been attributed to a reduction in sweating capacity, a decline in aerobic fitness, or a combination of the two. In a recent review of the effects of exercise and age on thermoregulation, Kenney and Gisolfi (1986) found no evidence that men or women up to 50 to 60 years of age had any impairment in temperature regulation that could be attributed to age per se. This conclusion is also supported by the review of Drinkwater (1986). Robinson et al. (1986) found a decrement in sweating capacity in men 44 to 60 years of age, but this decline in sweating had no adverse effect on the ability of these men to acclimatize to work in a hot-dry (50°C) environment. The decline in heat tolerance associated with men and women 50 to 60 years of age can be readily attributed to reductions in cardiovascular fitness, lack of heat acclimation, or both.

Sweat rate can be predicted from a measure of the overall heat load (E_req) and the maximal evaporative cooling capacity of the environment (E_max) (Shapiro et al., 1982). The advantage of the latter prediction is that sweat rate (and therefore water required) can be determined from environmental conditions, exercise intensity, and the type of clothing worn without making any physiological measurements (Shapiro et al., 1982). The formula for calculating sweat loss in g per m² per hour is

sweat loss = 27.9 × E_req(E_max)^-0.455

• Water requirements during exercise in the heat depend on fluid loss from sweating. Sweat rate is proportional to metabolic rate and can amount to 3 to 4 liters per hour or as much as 10 liters per day.

• Training and heat acclimatization can increase sweat rate by 10 to 20 percent or 200 to 300 ml per hour.