Acclimatization to Heat

Acclimatization is the physiologic adjustment made by the body to repeated or prolonged exposure to unaccustomed stress, producing increased tolerance to the stress. Acclimatization to heat is characterized by the following changes.

1. Improvement in the efficiency of the sweating mechanism. More sweat is produced but it contains smaller amounts of salt.

2. Improvement of temperature regulation with greater differ­ences in core and skin temperatures and smaller increases in rectal temperature with a given quantity of exercise.

3. Improved circulation with a decrease in heart rate.

4. Changes in the mechanisms that control salt loss through the sweat and urine.

5. Rapid response of the kidneys to dehydration by reduction of urine output.


Physically fit individuals adapt more quickly and completely to heat, becoming acclimatized in four to eight daily exposures. Some degree of acclimatization can occur in persons who are exposed to heat while remaining inactive. However, acclimatiza­tion is more complete in those who exercise during exposure to heat. Dehydration retards acclimatization, as does a diet that contains deficient or excessive amounts of salt.

Short daily exposures to heat are adequate to produce acclima­tization. The ideal exposure time for acclimatization is not clear, and probably depends on the duration of the heat stress for which the individual is being acclimatized (3 hour marathon versus 8 hour workshift). Exposures of less than two hours will not produce full benefits of acclimatization; nor will the same dura­tion of exposure if it is broken up into two or more working periods. One period of exposure is preferred.

Most of the acclimatization occurs in the first four to seven days. It is nearly complete after twelve to fourteen days of successive heat exposure. Acclimatization to heat is retained for up to two weeks after heat exposure has ceased. After one month, most of these adaptations to heat have been lost.

There is an appreciable mortality in young people subjected to hard physical exercise in hot, moist environments. This occurs primarily in military personnel, athletes, and laborers. Particularly virulent forms of heat stroke occur in young and fit individuals who are exercising vigorously in heat to which they are unac­climated.