Sweat and evaporation
Your body needs to keep its internal temperature constant in order to function properly. Even small changes of a few degrees in body temperature can have life-threatening results. Sweating results from your body trying to cool itself.
An average healthy adult loses from half a liter to a liter of sweat per day. During an intense workout or in hot environments, sweating can be increased to more than two liters per hour. Lost fluids must be restored by drinking sufficiently, since a fluid imbalance can again have bad consequences for your health.
Since there are a lot of opinions to the contrary, I want to repeat my understanding that sweating does not help you lose weight permenently. Although sweating does burn some calories, I do not believe it builds up to anything substantial. Any weight loss induced by sweating is, in my opinion, recovered as soon as you drink a glass of water.
To see how sweating works, let's start with some background information from the excellent (although expensive) book Biology, fourth edition by Neil A. Campbell, University of California, pp.899-900 .
"Heat will always be conducted from a body of higher temperature to one of lower temperature."
"If you were to sit in still air [at an air temperature of 23 degrees Celsius], conduction would account only for about 1% of your heat loss, convection for about 40%, radiation for another 50% and evaporation for about 9%. A breeze of just 15 km/h will increase total heat loss substantially by increasing convection fivefold. Evaporative cooling is increased greatly by the production of sweat."
In a hot, dry environment most of the sweat evaporates into the air. This helps cooling because evaporation consumes energy and hence draws heat from your skin.
"However, evaporation can only occur if the surrounding air is not saturated with water molecules (that is, the relative humidity is less than 100%). This is the biological basis fot the common complaint "the heat is not as bad as the humidity."
In the sauna
In the sauna, external heat causes the body to sweat in an attempt to control its internal temperature. Air humidity varies from 10% to 25%, depending on how much löyly is used. This is not high enough to block evaporation, but may lower it somewhat, causing the body to react by sweating even more. A 15-minute sauna can induce up to half a liter of sweat.
Although skin surface temperature may rise several degrees Celsius, internal body temperature is kept relatively unchanged. Prolonged sessions in the sauna may lead to a change of a degree or two. More in this article by Mikkel Aaland.
In a Turkish (steam) bath humidity can get near the saturation point of 100% relative humidity. The body sweats a lot trying to dissipate heat with perspiration but probably does not really succeed. Even so, much of the moisture on the skin in a steam bath is due to condensed water from the steam - not from sweat.
What about a bath
Notice that evaporation (and hence sweating) does not help heat transfer in a bath, since evaporation simply does not take place underwater. Campbell adds that "water is 50-100 times more efficient than air in conducting heat."
The water in a bath is in direct contact with the skin and if we don't lie very still, it is also in constant motion, thus increasing convection. Hence most of the heat transfer in a bath occurs due to water conduction and convection. (I am assuming that the body has no way to control the amount of energy dissipated though radiation.)
If the bath water is colder than body temperature, excess body heat is very efficiently transferred through condunction and convection. I assume that one would then not sweat since there would be no need for extra heat transfer by evaporation.
If the bath water is hotter than our body temperature, most of the energy transfer occurs from the water to the bather. Very little heat is transferred away from the body. Only the unsubmerged parts of your body will be able to dissipate heat through perspiration. I am unsure whether this means that the body does not sweat, but I'm taking a guess: the body keeps sweating even if it does not benefit from it. I do not see how the body would be able to perceive that sweating does it no good and I assume it would just keep sweating.
After all this I seize the opportunity to mention two things:
- Sauna is much more than just sweating, even though one does sweat a lot. The social surrounding and traditional aspects of the sauna make up for much of its relaxing effects.
- Infrared saunas are supposed to be the easiest route to sweating, but they lack the effects of convection and conduction found in traditional saunas. See my page on Infrared heaters for more information.