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 (WBGT_crit) and rate of rectal temperature change (vT_re) 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 WBGT_crit and vT_re. Twenty-eight non-acclimatized participants were divided into a HA (<i>n</i> = 15) and control (CON; <i>n</i> = 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 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 WBGT_crit after nine (28.5 ± 2.7°C vs. 30.5 ± 2.0°C; <i>P</i> = 0.016) but not 5 days (28.5 ± 2.4; <i>P</i> > 0.05). No effect of HA on vT_re was observed (<i>P</i> > 0.05). Four-to-eight days post-HA, WBGT_crit and vT_re did not differ compared with 9 days of HA (<i>P</i> > 0.05). However, a reduction in vT_re (-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 WBGT_crit and indicate that 9 days of HA proceeded by adequate recovery reduced vT_re during exercise in the heat.<b>NEW & NOTEWORTHY</b> We assessed the effect of heat acclimation (HA) on critical environmental limits and rate of rectal temperature change. We show that more than 5 days of heat acclimation are required to increase critical environmental limits and that 9 days of HA proceeded by adequate recovery reduces the rate of rectal temperature change. These findings enhance our understanding of heat acclimation's effect on work capacity in the heat and may be used to design occupational guidelines.