physiology

Why do you suffer the most from heat in hot and humid environments?

To understand why, with the same environmental temperature, in a warm-humid place the heat is more affected than in a dry place, it is necessary to go over the dispersion mechanisms of body heat.

In particular, it is necessary to consider the most effective one: the evaporation of sweat (for a liter of evaporated sweat we have the dispersion of about 600Kcal).

The amount of heat given off by the body by evaporation depends:

  1. from the body's ability to bring water to the surface, which in turn depends on:
    • amount of sweat produced;
    • extension of the sweaty area in relation to the total body surface;
  2. from the capacity of the environment to remove the water vapor produced by evaporation of sweat, which in turn depends on:
    • temperature and humidity of the ambient air;
    • extent of the flow of convective air currents on the body surface.

Point number 2 depends above all on the difference between the vapor pressure of the liquid (sweat) that wets the skin ( P H2O skin ), and the vapor pressure of the surrounding environment ( P H2O environment ). Therefore, an increase in environmental P H2O reduces the capacity of the environment to remove the water vapor produced by the evaporation of sweat.

The pressure (or tension) of water vapor in the environment is a function of temperature and relative humidity. The more the relative humidity increases, the more the P H2O increases, preventing the evaporation of sweat .

To better clarify the concept, a comparison can be made with the pressure cooker. Thanks to the hermetic seal, the pressure cooker holds the water vapor, increasing the P H2O of the air. The increase in pressure is opposed to the evaporation of the water contained in the pot, which for this reason will reach boiling at temperatures above 100 ° C (usually around 120 ° C), accelerating the cooking of food.

For all these reasons, if the air is dry and is moved by currents that facilitate the loss of heat due to convention and evaporation (fan effect), a healthy person can withstand for several hours environmental temperatures even higher than 60 ° C (even in the Finnish saunas are reached at 80-90 ° C without too many problems). On the other hand, when the relative humidity of the environment is very high, or when the body is immersed in water, temperatures above 30-35 ° C are already accompanied by an increase in body temperature.

If the air is saturated with water vapor, for example in a Turkish bath, there is no more gradient due to the evaporation of the sweat, so this does not evaporate. In these cases the sweat will tend to drip without evaporating, therefore not only will there be no heat dispersion (with an increase in the temperature inside the body), but there will even be dehydration due to the loss of liquids with sweat (produced in large quantities in a vain attempt to to cope with increased body temperature). This is why the sultry (hot-humid) climate is considered much more dangerous due to the risk of hyperthermia compared to a hot-dry and / or ventilated climate.