Heat Acclimatisation

The set of physiological adaptations produced by repeated heat exposure that let the body hold core temperature with less strain. Relevant beyond athletes as heatwaves become routine (western Europe had its third of 2026 by July).

Physiology

Repeated heat exposure:

• raises plasma volume — more stroke volume, lower heart rate for the same work;
• improves skin blood flow, so heat reaches the surface faster;
• lowers the sweating threshold — perspiration starts at a lower core temperature;
• reduces electrolyte loss per unit of sweat (relevant to Electrolytes You Probably Don't Need (Economist): acclimatised sweat is dilute).

Protocols

Protocol	Dose	Evidence	Note
Controlled hyperthermia (lab)	30 min exercise to core 38.5°C, then 60 min holding it; daily 5–7 days	Gold standard	Needs heat chamber + rectal thermometry
Exercise outdoors in heat	Not quantified	Expert opinion (Maxwell, Brighton)	"Naturally acclimatises you"
Exercise + hot bath	40 min treadmill at 18°C, then 40 min hot bath, daily × 6 days	Scand J Med Sci Sports 2016	Improved several acclimatisation measures; n not reported
Hot bath alone	1 h/day at 40°C	Healthcare, Apr 2026 (Wilson, Middlesex) — healthy adults > 65	Signs of acclimatisation after 4 days; n and design not reported in the article

Time course

• 75–80% of adaptation accrues in the first 4–7 days.
• Adaptation decays without continued exposure; it is not permanent.
• Acclimatisation reduces strain but is "not immunity": heat illness risk persists.

What is not established

• Whether passive (bath-only) acclimatisation changes real-world heat-illness or mortality outcomes — the studies measure physiological markers.
• Effect sizes and sample sizes for both home protocols were not reported in the source; the 2026 study population is narrow (healthy over-65s).

Related

• How to Train for a Heatwave (Economist) — the only source so far.
• Minimum Effective Dose of Exercise — a sibling "how little is enough" question, answered here in days of exposure.