Loss of heat acclimation and time to re-establish acclimation

Acclimation in a hot environment is one potent means to decrease the heat strain of work in a hot environment. However, with diminished heat exposure, positive adaptations of acclimation may be lost. This rate of loss is equivocal and, once established, could be used to prescribe the time for re-acclimation. The purpose of this study was to determine the rate of loss of heat acclimation over a period of 6 weeks and determine the time needed for re-acclimation after 2 weeks and 4 weeks of de-acclimation in ten healthy participants. All participants first underwent an initial acclimation period (a 3-day plateau in Tre was used to signify acclimation). Based on the mean time to acclimate in Phase 1 (mean time to acclimate = 6.1 ± 1.4 days), the loss of acclimation was mapped and participants were randomly assigned to one of two groups: one that underwent one 2-hr heat exposure at 1, 3, and 5 weeks post-acclimation, and one that underwent one 2-hr heat exposure session at 2,4, and 6 weeks. Complete loss of acclimation occurred in 6 weeks and, as expected, work HR and Tre increased with increasing time away from the heat (p<0.05). Based on the time for total loss of acclimation from Phase 1, participants in Phase 2 (n = 8) first underwent acclimation. Then, after either a 2-week or 4-week absence from the heat, participants returned to the laboratory for re-acclimation. While not statistically significant yet practically significant (p = 0.18; one-tailed confidence interval), average days for re-acclimation in the 2-week group tended to be fewer than in the 4-week group (days for re-acclimation = 3.8 ± 1.2 and 5.3 ± 1.9, respectively). Based on these general trends, for occupational settings, a re-acclimation period of 4 days is recommended after 2 weeks absence from the heat, 5 days for 4 weeks absence from the heat, and complete acclimation (6 days) after 6 weeks absence or more from the heat.