A Cooling Trend

The human body has numerous clever, if not always comfortable, ways to retool itself to tolerate heat waves.

July 27, 1999|By LINELL SMITH | LINELL SMITH,SUN STAFF

If you're convinced you just weren't made for a scorching, steamy summer ... think again.

"Humans are basically tropical animals," says Michael Sawka, chief of the thermal and mountain medicine division at the U.S. Army Institute of Environmental Medicine in Natick, Mass. "If given adequate water and shade, they can survive and work at almost any naturally occurring heat stress in the world."

But not, he adds, without a few physiological adjustments.

It generally takes between four and 15 days to acclimate to a dramatic increase in heat. How quickly you adjust depends partially upon how fit you are. Athletes and people who work outside generally need about an hour of exposure each day to regear their bodies into tropics mode.

The most important change occurs with the reprogramming of the body's cooling system.

Most comfortable when its temperature is around 98 degrees, your body has an ingenious way of adjusting its various systems in order to stay that way. Your cardiovascular system concentrates on distributing more blood than usual to the skin so that your body's excess heat can evaporate.

Before you adjust to the heat, you can often feel light-headed and crampy when exercising or working outdoors. The light-headedness comes from the body's attempt to cool itself by sending more blood to the skin and less to the brain and internal organs. The muscle cramping can be caused by excessive sweating that loses electrolytes, charged chemical particles.

As your body reprograms itself, however, your blood volume and pressure will stabilize. And the nature of your perspiration will gradually change. Even though you sweat more frequently, your body will lose less sodium and potassium than it does in cooler weather, says clinical exercise physiologist Erik Jaynes of Baltimore's Union Memorial Hospital. Similarly, your metabolism will decrease slightly to help keep your body from overheating.

When you are fully acclimated, your core and skin temperatures are less likely to overheat and your heart rate is less apt to rise from heat stress. These changes will free more blood to service your muscles when you exercise rather than diverting it toward cooling the body.

You will also begin sweating more efficiently and at a lower temperature than you do in winter, which means you must drink more water. The body doesn't register thirst until its fluid levels are already reduced. Judging from thirst alone, people tend to drink only a half to two-thirds of the fluids they need.

Exercise physiologists caution against skipping meals because you tend to consume most of your fluids then. (It's also a good idea to eat light in hot weather so that you won't burden your body with heavy digesting while it's working so hard to stay cool.)

Dehydration stresses the body by requiring the heart to beat faster to maintain blood pressure in the absence of adequate fluid.

Dehydration often occurs in climates that are hot and dry because sweat evaporates so effectively that people don't even realize they are perspiring. Baltimore's tradition of high heat and humidity, on the other hand, can prevent sweat from evaporating, which reduces its ability to cool the body.

"Every drop of sweat that falls to the ground is wasted," says Kerry Stewart, associate professor of medicine and clinical exercise physiologist at the Bayview campus of Johns Hopkins Medical Institutions. "Think of how when you pour water on a hot surface, it turns into steam and cools the surface by evaporating. Water that would immediately roll off a hot surface wouldn't have time to cool it down."

Those at great risk from summer heat waves include the elderly and infants, who cannot regulate their body temperatures very well; the very obese; and chronic alcoholics; Maryland's 10 confirmed heat-related deaths so far this summer involved people ranging in age from 2 to 90. Certain medications such as antihistamines and antipsychotics can make it more difficult to tolerate the heat as well.

Some symptoms of heat exhaustion are increasing fatigue, weakness and anxiety. Losing fluid by sweating will reduce the volume of blood and lower blood pressure. A person may feel faint when standing because the blood will collect in blood vessels of the legs, which are dilated from the heat.

Heat cramps, caused by an excessive loss of fluids and electrolytes in sweat, often begin in the muscles of the hands, calves or feet.

To help your body keep cool, dress in loose cottons and microfiber materials which let air pass through to your skin while also allowing for heat evaporation. Hats, long sleeved shirts and long pants will also keep you cooler by protecting your skin from direct sun.

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