Heat acclimation
Heat acclimation is the set of adaptive changes that the human body makes in response to repeated exposure to heat, and that make later sessions easier to endure. The adaptations concern mainly Sweating, the circulation and the kidneys, and they are the reason a regular bather sweats earlier and more copiously in the sauna, feels the heat less, and shows a smaller rise of Core body temperature and heart rate for the same session. The term is used interchangeably with heat acclimatisation, with the convention that acclimation refers to controlled laboratory or deliberate exposure and acclimatisation to natural exposure such as living in a hot climate.
The adaptations
The most systematic description of heat adaptation comes from the sports and occupational literature, where temperature-controlled protocols have been measured in detail. Repeated daily heat exposures of roughly an hour produce, over about one to two weeks, a characteristic set of changes:
- Higher sweat rate for the same core temperature, and an earlier onset of sweating;
- Lower salt content of the sweat, conserving sodium (see Electrolyte balance);
- A larger plasma volume, which supports both sweating and circulatory stability;
- Lower heart rate during heat exposure, with skin blood flow better maintained (see Blood circulation);
- A somewhat lower and later peak core temperature for the same stimulus.
The precise time course depends on the intensity and duration of the exposure: most of the change develops in the first week, and the adaptations decay over the weeks after exposure stops, so a person who has not bathed for a month is back near their starting point. Sports science summarises these effects for competitors who prepare for hot events, and the same physiology underlies the gradual conditioning of workers in hot environments.
Acclimation in the sauna
A Finnish-style sauna delivers a heat load that is largely passive — the bather sits still — so it is a gentler stimulus than exercise in the heat, and the training literature would not expect it to produce the full exercise-acclimation package by itself. Nevertheless, the basic components — repeated skin and core temperature rises, profuse sweating, and circulatory load — are the standard inputs for heat adaptation, and regular bathers show the corresponding signature: earlier, heavier, more dilute sweating and a less strained cardiovascular response. Session length and frequency are the levers usually recommended: a few sessions per week, long enough to produce sustained sweating, with fluid replacement, which itself assists the plasma-volume adaptation (see Sauna session duration). Acclimation is one of the benefits of regularity distinguished in Sauna research from the one-off effects of a single bath.
Several cautions attach to the idea of "getting used to the heat". Acclimation reduces but does not remove the risk of Heat stress and Heat exhaustion: acclimatised athletes, by sweating more, also lose more water, and their protection depends on drinking. Acclimation does not prevent the specific hazards of a sauna that are unrelated to cardiovascular conditioning, such as contact with a hot stove (see Burn prevention in a sauna). And acclimation acquired in a temperate country does not equal lifelong tropical residence, which involves larger and longer-lasting physiological adjustments; the modest form seen in bathers is simply an improvement against baseline.
Who benefits and who should be cautious
People who tolerate heat well — healthy adults at rest — typically develop the ordinary adaptations within a couple of weeks of regular bathing, and the changes are considered part of the normal physiology of the practice. For older adults, whose thermoregulatory responses are slower, the same gradual approach is even more important, but it should be undertaken cautiously because the cardiovascular strain of each session is not reduced by intent (see Sauna contraindications and Medical advice before sauna). Anyone with chronic illness, who is taking medication that affects the circulation or sweating, or who is pregnant should not use the sauna as a deliberate acclimation tool without clinical advice.
From the perspective of this encyclopaedia the practical summary is simple: heat acclimation is real, measurable and specific to the stimulus — it makes the bather sweat more effectively, keeps the pulse lower, and lowers the thermal strain of a given session; it does not make the room cooler, the safety rules different, or the hazards of overdoing it vanish. Its scientific basis, and the quality of the underlying studies, is set out in the articles on Sauna research and Sauna and health.
See also
References
- Périard JD, Racinais S, Sawka MN. "Adaptations and mechanisms of human heat acclimation: applications for competitive athletes and sports." Scandinavian Journal of Medicine & Science in Sports 25(S1):20–38, 2015. doi:10.1111/sms.12408
- Taylor NAS. "Human heat adaptation." Comprehensive Physiology 4(1):325–365, 2014. doi:10.1002/cphy.c130022
- Kukkonen-Harjula K, Kauppinen K. "Health effects and risks of sauna bathing." International Journal of Circumpolar Health 65(3):195–205, 2006. doi:10.3402/ijch.v65i3.18102
- Tansey EA, Johnson CD. "Recent advances in thermoregulation." Advances in Physiology Education 39(3):139–148, 2015. doi:10.1152/advan.00126.2014
- Laukkanen JA, Laukkanen T, Kunutsor SK. "Cardiovascular and other health benefits of sauna bathing: a review of the evidence." Mayo Clinic Proceedings 93(8):1111–1121, 2018. doi:10.1016/j.mayocp.2018.04.008
- Cheuvront SN, Kenefick RW. "Dehydration: physiology, assessment, and performance effects." Comprehensive Physiology 4(1):257–285, 2014. doi:10.1002/cphy.c130017