Across the thundering heat of an African savanna, a million years ago, give or take a few hundred centuries, runs something you would recognize instantly as almost human. It runs to live. Not fast, and not quick, it will run, with short breaks, for nearly 24 hours. In the end it will catch and kill a prehistoric antelope, using its bare hands as weapons.

In order to survive as a species, humans adapted in their own special ways, with virtually hairless skin filled with abundant sweat glands and powered by a cardiovascular system of marvelous endurance. Those same additions allow you spring and summer. You are not however, a foolproof design. Overheating can ruin your day, and your life.

To understand heat illness, let's start with the fundamentals of human thermal dynamics, body heat production vs. body heat loss. There are two major internal sources of heat. Even as you sit quietly reading these words, you are making heat via basal metabolism, the energy necessary to sustain your life at complete rest. When you leave the house and start exercising, a second heat source kicks in, exercise metabolism. Strenuous exercise metabolism may produce 15 to 18 times the amount of heat of basal metabolism, depending on your level of fitness. From external sources, the human body can absorb heat, from things such as the sun, a fire, and the ingestion of hot drinks. You end up with far more heat than you need and, if you couldn't shed the excess, you would literally cook in your own juices.

Human heat is lost in four ways: conduction, radiation, convection, and evaporation. Conduction, heat loss through direct contact with something cooler than you, does not help much on a hot day. On a desert-like afternoon, you may actually take in heat from a hot environment. Radiation is energy lost directly from your skin's surface and, as the air warms up around you, it may effectively stop on a sunny summer day. Convection is heat loss through the movement of air around your body. Without wind, convective heat loss practically stops when you're not moving, and it stops when the air temperature reaches approximately 92 degrees F.

That leaves evaporation, the vaporization of sweat from your skin, the primary source of heat loss for the human. As your skin heats up, pores dilate and sweat floods out. Evaporation of the sweat cools your skin, heat is drawn from your blood near the surface of your body, and the cooler blood circulates to keep your insides maintained at an acceptable temperature. In other words, if you don't sweat, you ain't gonna make it.

Sweat comes from your circulatory system, and it's not uncommon to sweat out a liter of water in an hour during periods of exercise in a hot environment. This water loss may reach two-and-a-half liters per hour with prolonged exercise. And sweat contains salt, a critical component of normal body function. It is this combined water and electrolyte depletion that forms the basis of a spectrum of problems with one general name: heat illness.

Although you are unique in the animal world in your ability to shed excess heat, if body heat production gets a jump on body heat loss, you are heading toward heat illness. On the minor end of the problem are heat cramps, a painful spasm of major muscles that are being exercised. Those most often cramped are people unacclimatized to heat who are sweating profusely. Heat cramps are poorly understood, but probably result not only from the water lost in sweat, but also the salt lost in sweat. Gentle massage and stretching of the affected muscles usually provides relief. Drinking water, preferably with a pinch of salt per liter added, is advisable. Heat cramps do not often occur in someone who is adequately hydrated. Once the pain is gone, exercise may be continued if necessary, but a day of rest is better.

Prolonged sweating may move you along the spectrum to heat exhaustion, characterized by headache, dizziness, nausea, rapid breathing, and, of course, exhaustion. Sufferers are so sweaty they often feel cool, grow goose bumps and complain of chills. Treatment should include moving the exhausted person to a shady spot and oral rehydration with cool, very slightly salty water. Some experts prefer using an electrolyte-balanced drink such as Gatorade, but the drink should be watered-down three or four times for more rapid absorption in a resting person. Maximum absorption ranges from 150 to 250 ml per 15 minutes, so it takes about an hour to get a liter back into circulation. Heat exhaustion is not physiologically damaging, but it should be treated aggressively before it progresses to a more serious condition.

On the serious end of the spectrum lies heat stroke, a problem that kills approximately 4000 people in the United States every year. There are two varieties of heat stroke. In classic heat stroke, the patient is usually elderly or sick, or both. Temperature and humidity have been high for several days, and the patient has dehydrated to the point where his or her heat loss mechanisms are overwhelmed. You might say they simply run out of sweat. Skin gets hot, red, and dry. They lapse into a coma and, if untreated, die.

But more and more people are being killed of the second variety, exertional heat stroke. The victim is usually young, fit, and unaccustomed to heat, sweating but producing heat faster than it can be shed. Signs include, primarily, a sudden and very noticeable alteration in normal mental function: disorientation, irritability, combativeness, bizarre delusions, incoherent speech. Skin is hot and red, but wet with sweat. Rapid breathing and rapid heart rates are almost universal. Collapse is imminent.

Quick cooling may be required to saver this victim's life, and the best method includes removal of clothing, covering with wet cotton, and vigorously fanning, all of which increases evaporative heat loss. Massaging of arms and legs and ice packs at the neck, groin, and armpits increase heat loss. Throwing patients into cold water is less effective and often dangerous since they are difficult to manage and may drown. Heat stroke victims should be seen by a physician as soon as possible, even if they seem to have recovered. Too much internal heat can cause breakdowns in some body systems that show up later.

ACCLIMATIZATION TO HEAT

• Acclimatization usually takes 2 to 3 weeks.
• Exercise in the early morning and late evening for the first week.
• Work your way gradually toward midday exercise.
• On spectacularly hot days, only exercise early and late.
• Drink one-fourth liter of cold water every 15 minutes during intense exercise.
• Drink at least three liters of water every day.

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