The effect of heat acclimation on critical environmental limits and rate of rectal temperature change.

Abstract

Quantifying the effect of heat acclimation (HA) on critical wet-bulb globe temperature (WBGT_crit) and rate of rectal temperature change (vT_re) is relevant for developing guidelines with regards to occupational safety whilst working in warm environments. This study quantified the effect of HA and the period following cessation of the HA-protocol on WBGT_crit and vT_re. Twenty-eight non-acclimatized participants were divided into a HA (N=15) and control (CON; N=13) group. The HA-group underwent a warm-humid (35°C, 65% relative humidity) controlled hyperthermia HA protocol (5-9 days of achieving T_re^~ 38.5°C for 60 min) and four progressive heat stress tests (HSTs) to identify WBGT_crit and examine vT_re: pre-, after five and nine days of HA, and four-to-eight days of no heat exposure following HA. CON performed two HSTs on average 13 days apart without heat exposure in between. HA increased WBGT_crit after nine (28.5 ± 2.7°C vs. 30.5 ± 2.0°C; P = 0.016), but not five days (28.5 ± 2.4; P > 0.05). No effect of HA on vT_re was observed (P > 0.05). Four-to-eight days post-HA, WBGT_crit and vT_re did not differ compared to nine days of HA (P > 0.05). However, a reduction in vT_re (-0.4 ± 0.3°C/ hour) was observed when comparing four-to-eight days post-HA to pre-HA. In conclusion, our results demonstrate that more than five days of HA are required to increase WBGT_crit and indicate that nine days of HA proceeded by adequate recovery reduced vT_re during exercise in the heat.