As they say, horses sweat, men perspire, and ladies glow. Why does the body sweat? Sweat is the body's air conditioning system. Heat is carried from the body core to the skin by the blood. Sweat evaporates from the skin and cools the blood . The purpose of perspiration is to maintain normal body heat. When the body gains heat from the environment or from physical labor, sweat is expended to bring the body temperature down.
It would seem that if water is limited, survival in the desert would depend on water conservation. This is not entirely correct. Most people define water conservation as rationing water--say to one cup per man per day. This is not recommended procedure. In the survival situation, water should not be wasted, but it should not be hoarded either. Studies have shown that the best course is to drink a few ounces of water every fifteen minutes to satisfy thirst. The water in the body will keep the body temperature down, not the water in the canteen. People have been found dead of thirst with full canteens.
The hotter a person becomes, the more sweat is required to cool off. As long as this water can be replaced, sweating is an efficient cooling system. Humans have been shown to survive temperatures of 260 degrees F when supplied with unlimited water. If water is scarce, too much sweat can be hazardous to the health. Rather than water conservation, one should speak of sweat conservation.
Sweat is produced by the body to keep the body temperature within the normal range. By reducing the amount of heat the body is exposed to it is possible to reduce the perspiration rate. This is what is meant by sweat conservation--saving the water already in the body by controlling the amount which is lost through sweating.
The body temperature rises as a result of four factors:
Reducing the amount of heat gained through these four sources can reduce the water lost as sweat.
Heat is gained through radiation by direct exposure to the sun's rays and by exposure to reflected heat from the ground. An obvious solution to radiation heat gain is to move into the shade. A person sitting quietly in the shade uses about one half as much water as a person sitting in the sun. The deeper the shade the more effective this method is.
Wearing clothing will reduce the amount of radiation which reaches the skin. Most people consider shorts and a sleeveless shirt to be high fashion in hot weather, but those who live in the desert wear full, loose clothing and expose as little skin to the sun as possible. Consider the Arabian Bedouin. His costume consists of a long burnoose (a loose fitting robe) of wool, usually worn over a long-sleeved shirt, a jacket, and another burnoose.
A hat should always be worn in the desert. The head is exposed to the direct rays of the sun, and the many capillaries in the scalp disperse this heat through the body quickly. The best sort of hat has a brim to protect the ears, face, and neck and a high crown to provide dead air space directly above the head for more insulation. Tucking a bandanna under the hat, a la Beau Geste, will protect the neck. If no hat is available even a piece of cloth tied over the head is infinitely better than nothing.
Convection can be thought of as air currents. Hot air which comes in contact with the skin will result in more sweating. The air in a shaded area is cooler than the air in a sunny area. This is another good reason to move into the shade. If a vehicle is available it will provide shade, but the interior heats up due to the greenhouse effect. Heat enters through the glass of the windows and is trapped. During the heat of the day one should sit in the shade beside the car, not inside.
Convection is a factor in sweat evaporation. Wind can be thought of as air currents, and exposure to wind speeds up the evaporation process. It feels wonderfully cool to stand in a breeze, but in a survival situation the air is robbing the body of sweat which cannot be replaced. One should stay in still air and avoid the wind.
Conduction is the transfer of heat between objects of different temperatures. When a warm object is placed next to a cool object, the warm object becomes cooler and the cool object becomes warmer. Heat gain through conduction occurs when the body is in contact with a hotter surface. If possible one should not sit on the ground. The ground temperature can be 70 degrees F hotter than the air temperature just a few inches above the surface. One should not sit on hot rocks or other dark surfaces which absorb heat. If a cooler surface is found, such as damp sand, one should lie down and take advantage of it.
In short, whenever possible find a surface which is cooler than the body to sit or lie on. Avoid contact with a surface which is hotter than the body.
The body metabolism increases with exercise. When the body is working it produces heat as a by-product. Everyone has experienced a rise in body heat while doing hard work. As the body heats up, more sweat is expended to bring the temperature back to normal. The obvious solution is to sit still. A person sitting in the shade uses one quarter as much water as a person walking in the sun . The following charts summarize expected survival time while resting or walking in different temperatures:
Days of expected survival in the desert under two conditions*
| Available Water Per Person, U. S. Quarts |
| Max Temp in Shade F | 0 | 1 | 2 | 4 | 10 | |
| 120 | 2 | 2 | 2 | 2.5 | 3 | |
| 110 | 3 | 3 | 3.5 | 4 | 5 | |
| 100 | 5 | 5.5 | 6 | 7 | 9.5 | |
| 90 | 7 | 8 | 9 | 10.5 | 15 | |
| 80 | 9 | 10 | 11 | 13 | 19 | |
| 70 | 10 | 11 | 12 | 14 | 20.5 | |
| 60 | 10 | 11 | 12 | 14 | 21 | |
| 50 | 10 | 11 | 12 | 14.5 | 21 |
| Available Water Per Person, U. S. Quarts |
| Max. Temp. in shade F | 0 | 1 | 2 | 4 | 10 | |
| 120 | 1 | 2 | 2 | 2.5 | 3 | |
| 110 | 2 | 2 | 2.5 | 3 | 3.5 | |
| 100 | 3 | 3.5 | 3.5 | 4.5 | 5.5 | |
| 90 | 5 | 5.5 | 5.5 | 6.5 | 8 | |
| 80 | 7 | 7.5 | 8 | 9.5 | 11.5 | |
| 70 | 7.5 | 8 | 9 | 10.5 | 13.5 | |
| 60 | 8 | 8.5 | 9 | 11 | 14 | |
| 50 | 8 | 8.5 | 9 | 11 | 14 |
One should readily see that the expected days of survival can be doubled in many circumstances by sitting quietly in the shade. Notice that there are no figures available for days of survival while walking in full sun. In extremely high temperatures, survival time exercising in full sun with no water is limited to hours.
These tables reinforce the fact that water should be consumed and not hoarded. A person without water can survive for two days at a temperature of 120 degrees. Two quarts of water do not increase the expected survival time. The sweat output under these circumstances is simply too great. It is best to renew the body's supply of water by drinking enough to satisfy thirst. Drinking more water does not result in more sweat. Sweating occurs only to cool the body, and if the water is not supplied by drinking, the body will rob its own tissues, then its own blood for the water it needs to maintain body temperature. As the blood becomes thicker, it is less efficient transferring heat from the body core to the skin where it can be dissipated. The body core temperature begins to rise, and the desert claims another victim.
Moist air lost through the lungs also serves to cool the body. This water loss through evaporation can be reduced by breathing through the nose, not talking, and not smoking.
Clothing will trap the sweat next to the skin and reduce the amount lost to the dry air of the desert. Clothing should be loose and permeable to allow some evaporation. If sweat was not allowed to evaporate at all, the body's cooling system would break down and death by heat stroke would follow rapidly. Bare skin, however, will allow sweat to evaporate as soon as it reaches the surface. This is a waste of sweat.
Digestion of food requires one pound of water (about one pint) per pound of food. In a survival situation where water is limited, one should. not eat. Even moist foods will rob the body of more water than they supply. The body treats milk as a food.
Alcohol is a food, and the body requires water to digest alcohol. Alcohol will impair the thinking process (especially on an empty stomach!) and the ability to reason clearly is the single most important factor in survival. Alcohol is a diuretic, in that it causes more water to be passed through the kidneys. One should not drink alcoholic beverages in a survival situation.
Salt tablets should not be taken unless there is sufficient water available. Salt will dehydrate the body rapidly.
Urine, blood, or salt water should not be drunk. The salt in urine and in blood will dehydrate the body. These liquids may be used to wet the clothing and cool the body.