Summertime sizzlers.

The human body is equipped with an amazingly efficient thermoregulatory system that keeps your body temperature tightly regulated at close to 98.6 degrees Fahrenheit under a wide range of environmental conditions. The hypothalamus, an area in the front of the brain, is responsible for this job along with water balance, sleep cycles, appetite, and sexual function. When the, circulating blood exceeds 98.6[degrees], the hypothalamus signals the heart to beat harder and faster, the blood vessels to expand at the skin's surface to help radiate heat, and sweat glands to kick into gear to cool the skin by evaporation.

When it is very hot and muggy the job gets a whole lot harder. Sweat does not evaporate as fast when the relative humidity is high and heat does not radiate from the skin quickly when there is too little gradient between skin temperature and air temperature. Add exercise to the scenario in which your muscles are generating 20 times more heat than they do when they are resting and it is easy to understand how you can dangerously overheat.

When your body cannot keep up with cooling you down, your body temperature starts to rise and heat stress can progress to heat exhaustion or life threatening heat stroke. Symptoms include headache, dizziness, muscle cramps, fatigue, and nausea. A symptom of impending trouble can be, ironically, a loss of your perception that something is wrong. Heat stroke, when the heat overwhelms your ability to keep cool, follows. Core temperatures soar, you may become delirious and lose muscle control. Respiration and heart rate is rapid. Blood pressure can drop and coma ensue. It is a life threatening emergency requiring medical care. Although children and the elderly are at greater risk of heat stroke because their bodies are less efficient at the cooling process; young, healthy, fit adults are also vulnerable. When the weather starts to sizzle, you must protect yourself.

Heat stress can also be a problem early in the season when the weather may not be as miserably hot as it will be later in the summer. You may not have fully adjusted to the heat with a process called acclimation. When your exercising body is exposed to heat over time, usually about two weeks, physiological changes occur that improve your ability to stay cool. Once acclimated, you lose less sodium in your sweat, there is an increase in the volume of sweat you produce and the rate at which you sweat, and you start sweating at a lower core temperature. Overall, your thermoregulatory processes become more efficient and the cardiovascular strain caused by heat stress is less. But before your body has had a chance to make these changes, it won't take a heat wave to produce heat stress.

Use common sense and listen to your body. If you're feeling sick, it is a warning. Here are some recommendations to bear in mind throughout the summer season.

* Run in the early morning before the sun is up.

* Wear light colored clothing designed to allow for maximum cooling. Runners can benefit from advances in athletic clothing in recent years. Choose shirts, shorts, shoes and socks that wick moisture efficiently.

* Drink a lot before, during, and after exercise.

* Choose a sport drink to replace sodium and electrolytes.

* Get acclimated before you take on long or hard workouts in the heat.

* When it's very hot, reduce exercise intensity and duration.

* If you feel sick or light headed, get out of the sun, drink cool liquids, and cool down.

* Take a cool shower or a swim before your run. Research has shown that you can run longer and harder in heat if you are pre-cooled. There are also commercially available cooling devices to wear around your neck for pre-cooling.

(Medicine & Science in Sports & Exercise, 1999, Vol. 31, No. 2, pp. 224-228; The Lancet, 2000, Vol. 355, No. 9203, pp. 569-571 and No. 9219, pp. 1993-1994; Military Medicine, 2000, Vol. 165, No. 6, pp. 500-503)