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

Quantifying the effect of heat acclimation (HA) on critical wet-bulb globe temperature (WBGTcrit) and rate of rectal temperature change (vTre) is relevant for developing guidelines with regards to occupational safety while working in warm environments. This study quantified the effect of HA and the period following cessation of the HA protocol on WBGTcrit and vTre. 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 Tre _38.5_C for 60 min) and four progressive heat stress tests (HSTs) to identify WBGTcrit and examine vTre: pre-, after 5 and 9 days of HA, and 4 to 8 days of no heat exposure following HA. CON performed two HSTs on average 13 days apart without heat exposure in between. HA increased WBGTcrit after nine (28.5 ±2.7_C vs. 30.5 ± 2.0_C; P = 0.016) but not 5 days (28.5 ± 2.4; P > 0.05). No effect of HA on vTre was observed (P > 0.05). Four-to-eight days post-HA, WBGTcrit and vTre did not differ compared with 9 days of HA (P > 0.05). However, a reduction in vTre (_0.4 ± 0.3 _C/h) was observed when comparing 4 to 8 days post-HA to pre-HA. In conclusion, our results demonstrate that more than 5 days of HA are required to increase WBGTcrit and indicate that 9 days of HA proceeded by adequate recovery reduced vTre during exercise in the heat.