There is no doubt that sweating, changes in body temperature and changes in circulatory function are main physiological changes induced by heat exposure and that these physiological changes may be used to assess a so called “heat tolerance” of each person. It will be natural to inquire what combination of these physiological changes is the best index for the evaluation of “heat tolerance”.
It may be presumed that there is an underlying factor which is called as “heat tolerance” and that this factor controls various physiological changes induced by heat exposure simultaneously. If this hypothesis is true, the factor which may be expressed as a transformed variate of various physiological changes must be the best index for the evaluation of “heat tolerance”.
The above hypothesis has been tested by applying a statistical method called “principal components analysis” to the data on several physiological changes when young healthy male subjects were compelled to exercise in a warm bath.

In order to get basic data for preventing heat stroke accident during exercise in a hot environment, we analysed the relation between environmental temperature, and drinking and sweating. We also analyed the effect of water intake on body temperature regulation during exercise.
The environmental temperature started to rise in April, reached the maximumin August, and then decreased. Water intake and sweating increased significantly with increase in WBGT, but there was no correlation between weight loss and WBGT. The rise in body temperature during exercise (0.52±0.080°C) was constant and independent of WBGT. The rise in oral temperature during exercise was affected by the water intake and it was significantly higher when water was not supplied than that with water supply (p<0.001) . Sweat rate was significantly greater when water was supplied than when it was not supplied (p<0.01) .
The above results suggest that the amount of water intake increased with the increase in WBGT, which guarantees the increase in sweating and as a result maintenance of constant oral temperature.
Therefore it is suggested that it is better to supply water during exercise to facilitate evaporative heat loss, which prevent rise in oral temperature.