REF-3489 · van den Bogaard, Klous, Cottle, Van Erp & Daanen, 2025 · Journal of Applied Physiology, · Paper
Five days of heat acclimation is not enough to raise the critical environmental limit
Nine are; and the drop in core-temperature rise appears only after 4-8 days of recovery, not during the block
Participants
n=28 non-acclimatized participants — heat-acclimation group n=15, control n=13.
Protocol
HA: 5–9 days of warm-humid controlled hyperthermia — 35 °C, 65 % relative humidity, holding rectal temperature (Tre) ≈38.5 °C for 60 min per session.
Four progressive heat-stress tests to find the critical wet-bulb globe temperature (WBGTcrit) and the rate of rectal temperature change (vTre): before HA, after 5 days, after 9 days, and after 4–8 days of no heat exposure following the block.
Control: two tests on average 13 days apart with no heat exposure between.
Key Findings
WBGTcrit rose only after NINE days — 28.5 ± 2.7 → 30.5 ± 2.0 °C (P = 0.016). After five days there was no change (28.5 ± 2.4 °C, P > 0.05).
No effect of HA on the rate of core-temperature rise during the block (P > 0.05).
4–8 days after the block, WBGTcrit and vTre did not differ from the 9-day values (P > 0.05) — the gain held over that window.
But compared with pre-HA, vTre was lower by 0.4 ± 0.3 °C/h at 4–8 days post-block — the authors' reading: 9 days of HA followed by adequate recovery reduced vTre.
Authors' conclusion: more than 5 days of HA are required to raise WBGTcrit; the findings are intended for occupational guidelines on work in the heat.
Practical Application
In 28 non-acclimatized participants, 5 days of controlled-hyperthermia heat acclimation did not raise the critical environmental limit, 9 days raised it by 2 °C WBGT (28.5 → 30.5 °C), and a lower rate of core-temperature rise versus baseline appeared only 4–8 days after the block ended. Budget more than five days of heat if the goal is to tolerate a hotter environment.
Inference, not tested: do not butt the heat block against the start. The reduction in core-temperature rise showed up 4–8 days after the last exposure; leaving that gap mirrors how a taper lets a hard block express itself.
Inference, not tested: early comfort in the heat is not proof the ceiling has moved — in this study the ceiling had not moved after 5 days.
Limitations
Population is occupational, not athletic: non-acclimatized participants in a study designed for occupational safety guidelines. Inference, not tested: trained athletes start from a higher baseline and may need fewer days; the day-counts here are not a prescription for them.
The outcome is a safety limit, not performance. WBGTcrit is the environment at which heat balance fails — it is not a time trial. Nothing here shows that nine days of heat makes anyone faster, and it was never designed to.
Small groups (15 / 13); the abstract does not say whether allocation was randomised.
The post-block window was 4–8 days; the study cannot say when inside that window the change appears, nor how long it lasts.
How this entry was checked and changed
Content correction 2026-09-27
Removed: "+2.0 °C sits at the bottom of the 2.0–3.0 °C WBGT allowance of ISO 7243", "HR, skin and core temperature adapt inside ~5 days, sweat loss takes 7–8", "5-day block buys cardiovascular comfort", WBGTcrit "by visual inspection" — none in the abstract.
Added the abstract's finding that WBGTcrit and vTre 4–8 days post-HA did not differ from the 9-day values; the vTre drop is versus pre-HA only.
"Recreationally active, low metabolic rates" and "athletes acclimatize faster" -> not in the abstract; tier restated, the athlete claim labelled as inference; lead now carries the numbers.
Cross-reference to REF-002 "48-h degradation rule" removed (REF-002 no longer carries it).