Skin temperature
Skin temperature is the temperature of the surface of the body, measured on the skin rather than at the interior. It is the temperature that a bather feels, that governs the exchange of heat with the surrounding air, and that rises sharply inside a sauna, where it is coupled to Thermoregulation of the body as a whole but behaves very differently from Core body temperature.
Skin temperature and the core
The body keeps the deep organs at a nearly constant temperature while the skin is deliberately allowed to run cooler or warmer, because the difference between the two drives the exchange of heat with the environment. At rest in a comfortable room the average skin temperature is roughly 32–34 °C, some three to five degrees below the core. The gradient matters more than either value alone: if the skin is warmer than the air, the body loses heat by radiation and convection; if the skin is cooler than the air, it gains heat. Vasodilation of the skin vessels raises skin temperature and increases the loss of metabolic heat, whereas vasoconstriction does the opposite.
Every heating and cooling input acts on the skin first. In a sauna the air is far hotter than the body surface, so the skin is heated directly by convection and by radiation from the hot stove and walls, and it also gains heat from the deeper tissues, where blood flowing to the surface brings internal heat with it. Skin temperature is therefore the fastest-changing of the body's temperatures during a bath: it begins to climb within seconds, well before the core follows. Because the sensation of warmth depends mainly on skin temperature, the bather judges the room through the skin while the safety-relevant quantity is the slow rise of the deep-body temperature.
Skin temperature during sauna bathing
In a Finnish-style sauna at 70–100 °C, the skin of a seated bather warms rapidly and typically stabilises some way below the air temperature at values around 40 °C and above, the exact level depending on the air temperature, the humidity and the blood flow through the skin. The rise is much larger than the one-degree-or-so change seen in the core, and it is this large surface-to-air gradient that allows the sweat film on the skin to evaporate and carry heat away; evaporation cools the skin itself, which in turn cools the blood passing beneath it. High humidity, such as after water has been thrown on the stones, slows evaporation and leaves the skin hotter and wetter for the same heat load.
Skin temperature falls again as soon as the bather leaves the hot room. In the cooling room or when swimming, the same surface adjusts quickly to the new conditions, and the familiar "glow" after a sauna reflects flushed, dilated skin vessels rather than an elevated core. Measurements and research uses are described under Temperature measurement in a sauna and Thermal imaging of a sauna; room instruments such as a Sauna thermometer or Sauna temperature sensor record the air, not the skin, and cannot substitute for contact or infrared measurement of the body surface.
Measurement
Skin temperature is usually measured either with small contact sensors (thermistors or thermocouples taped to the skin) or, in research, with infrared thermometry and thermography, which read the radiation emitted by the surface without touching it. Contact sensors are accurate at the point of attachment but are affected by the insulation of the tape and by local blood flow; infrared devices measure a patch of surface at a distance and can map temperature across the body, which is how skin reactions during bathing are studied. Simple temperature-sensing wristbands and similar consumer devices estimate skin temperature but generally report a derived value rather than a calibrated surface measurement.
Because the skin is the interface with the environment, measurements taken immediately after leaving the hot room reflect the room, not the body. There is no practical value in measuring skin temperature to decide whether it is safe to re-enter a sauna; the useful indicators are the bather's symptoms, such as dizziness, headache or nausea, which point to Heat stress and possible Heat exhaustion even when the surface is merely warm.
Sensation and thresholds of injury
The skin's temperature sensors are most sensitive to the rate of change: rapid warming, as at the start of a session, feels hotter than a slow rise to the same value. This is one reason that a bather may find a given bench temperature comfortable for ten minutes yet uncomfortable within the first minute of a hotter session; Skin temperature also explains why a change of seat position, moving the skin against a warmer or cooler surface, immediately changes the perceived temperature.
Sustained high skin temperatures injure the tissue. The modern modelling literature on burn prediction estimates that prolonged exposure of the skin to temperatures above roughly 43–44 °C begins to cause irreversible thermal damage, with the permissible time shrinking rapidly as the temperature rises: what takes hours just above 44 °C takes only seconds at hotter temperatures. In the sauna this matters in two separate ways. Exposure of the skin to the hot air in normal use is uncomfortable long before it is damaging, which is protective. Contact with solid hot surfaces is different: benches in long use, the stove surround, the heater stones at several hundred degrees and metal fittings conduct heat rapidly, and injuries occur where air would be tolerable — see Sauna burns and Burn prevention in a sauna. Small children have thinner skin and heat more quickly, which is why their skin temperature rises faster and shorter sessions are recommended (see Children and sauna).
Immune and skin-health consequences of heated skin are discussed under Skin and sauna, and the physiological context of the heat load under Sauna and health and Sauna research.
See also
References
- Tansey EA, Johnson CD. "Recent advances in thermoregulation." Advances in Physiology Education 39(3):139–148, 2015. doi:10.1152/advan.00126.2014
- 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
- Heinonen I, Laukkanen JA. "Effects of heat and cold on health, with special reference to Finnish sauna bathing." American Journal of Physiology – Regulatory, Integrative and Comparative Physiology 314(5):R629–R638, 2018. doi:10.1152/ajpregu.00115.2017
- Abraham JP, Plourde B, Vallez L, et al. "Estimating the time and temperature relationship for causation of deep-partial thickness skin burns." Burns 41(8):1741–1747, 2015. doi:10.1016/j.burns.2015.06.002
- Hussain J, Cohen M. "Clinical effects of regular dry sauna bathing: a systematic review." Evidence-Based Complementary and Alternative Medicine 2018:1857413, 2018. doi:10.1155/2018/1857413
- 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