Graham, J. F., & Rehrer, N. J. (1997). Decay and reinduction of heat acclimation. Medicine and Science in Sports and Exercise, 29(5), Supplement abstract 561.

The effects of heat acclimation, its decay, and subsequent reacclimation were investigated. Acclimation lasted 6 days, the exercise performed being 50 min of 70% VO2max cycling in 34 degrees Celsius with 70% relative humidity. This was followed by 21 days of decay (moderate temperatures) and a further 6 days of reacclimation as in the first exposure.

All variables changed over time. During the first acclimation, final exercise heart rates decreased from 182 bpm to 164 bpm on day 6. Rectal temperature decreased from 39.33 to 38.82 degrees Celsius and sweat rate increased from 1.30 to 1.64 L/hr. In the decay period, heart rate and sweat rate returned to previous levels but rectal temperature showed no alteration. In the final 6 days of reacclimation values returned to levels similar to the first adaptation.

Implication. Full acclimation to high heat and humidity occurred in 6 days. Decay occurred in heart rate and sweating but not in rectal temperature. Re-introduction to the hot-humid environment returned values to the original acclimation level but not beyond. There is no enhanced tolerance gained through multiple exposures to heat and humidity.

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