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How Does Heat Kill You?Your whole body stops working.

By Daniel Engber
Posted Friday, Aug. 4, 2006, at 6:23 PM ET

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The national heat wave may soon be over, but several hundred people have already died from "heat-related" causes. More than 160 people in California and at least 27 people in the eastern United States perished in the high temperatures. How does heat kill you?

Let us count the ways. A heat wave can kill you directly by inducing heatstroke, which damages the brain, the kidneys, and other organs. Or it can increase your chances of succumbing to a heart condition, a stroke, or breathing problems. Heat-related stress can kill you in other ways, too: Cocaine users, for example, are more likely to die from an overdose on a hot summer day.

Problems start when your body can't keep its core temperature close to 98.6 degrees. In general, your nervous system starts working to cool you down every time your central temperature rises a couple of degrees above normal. To do that, it tries to divert blood away from your internal organs and toward your skin. Since blood carries a lot of heat, your best chance of cooling off is to get that hot stuff pumping to the surface. Meanwhile, your sweat glands start to release water, which has a cooling effect as it evaporates from the surface of the skin.

On a very hot, sticky day, you may not be able to radiate any heat from the surface of your skin, and your sweat won't evaporate fast enough to keep you cool. (You'll have an even bigger problem if you're dehydrated.) The heart responds by pumping more blood away from the internal organs. This deprives the intestines of oxygen, which damages their linings and makes them more permeable to endotoxins. (The strain on the heart can also lead to arrhythmia or cardiac arrest.) At the same time, an overheated core causes an inflammatory response throughout the body. The combination of the inflammatory response and the endotoxins in the bloodstream can suppress the body's natural mechanism for cooling down.

Once your core gets above about 104 degrees, you're in serious danger. High internal temperatures lead to increased pressure in your skull and decreased blood flow to your brain. (Doctors diagnose "heatstroke" when the heat starts to affect your central nervous system.) Damaged tissue may also enter your bloodstream and lead to kidney failure. Very high internal temperatures—like 120 degrees—can destroy the cells in your body through direct heat damage.

Many factors increase your risk of succumbing to heat-related illness. The elderly have trouble regulating their body temperature because their circulatory systems aren't very efficient. They're also less mobile, which makes it harder for them to go somewhere cooler or get a glass of water. Small children have a very slow sweat response, and their hearts aren't always strong enough to cool their bodies. Obese people can't dissipate heat very quickly and may have fewer sweat glands in the skin that lies on top of their fatty tissue. Mentally ill patients (and elderly people) sometimes take drugs that inhibit perspiration and make it harder for them to cool down.

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Daniel Engber is an associate editor at Slate. He can be reached at .
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