Water loss and body temperature

The ability to sweat increases with repeated exposure to high temperatures. The highest sweating rates are attained by a person acclimatised through prolonged heat exposure. You lose more water, not less, as you acclimatise, so water saving comes from understanding the environment and avoiding activity during hot periods.

Safe body temperatures are maintained by the evaporation of sweat from bare skin, so cooling is not as efficient via sweat-soaked clothing. On a really hot day water lost through sweating may reach 1 litre/hour. Table 16.1 gives an indication of the minimum daily water requirement to maintain body fluid when resting in shade at a given temperature. Walking may double this water requirement.

A slight increase in body temperature is not unusual among people exerting themselves in the hot sun. A body temperature of 38°C or even 39°C can be tolerated, but higher temperatures can lead to serious injury or death. Once air temperature rises above skin temperature, heat loss can only be achieved by sweating.

Table 16.1 Minimum daily water requirements (resting in shade)
Mean temperature (oC)20253035
Water requirement (litres)1.21.42.55.1