Thursday, July 28, 2005

We're having a heat wave

The headline in the Philadelphia Enquirer says, "It's not the heat . . . actually, yes, it is." But actually, "No, it Isn't." The old adage, "it's not the heat, it's the humidity" is much more correct.

Heat and temperature are not the same thing. Without getting into thermodynamics, temperature measures the intensity of thermal energy while heat measures its quantity. You can think of the heat content of your body as being like a bathtub. The bath water is at body temperature (98.6 degrees F.), while the surrounding environment is usually cooler. If more heat enters the tub than leaves it, the temperature will rise. Thus even on a day when the outside temperature is in the low nineties, your body naturally loses heat. The problem is, it also gains heat. Besides contact with other objects, another source source of thermal energy is from other objects that radiate heat (like the sun or a furnace or just the surrounding environment). Since a body radiates heat in amounts proportional to its temperature, you usually lose heat to the environment by radiation faster than you gain it by that means, but extra hot objects (like the sun) can actually produce a radiant heat load that heats you up (hence you feel cooler in the shade). You also produce heat by metabolism and muscular actions, enough heat in fact, that you can maintain body temperature despite the fact that your surrounds are almost always cooler.

But what happens if the surrounding environment is actually warmer than body temperature? You can't lose heat by contact with a warmer environment or by radiating your own heat to it (because it is radiating even more heat back to you). A fan won't work because it is just blowing warmer air past you and helping you gain heat faster. Fortunately, there is one remaining way to lose heat, and that's by turning body water into water vapor. This makes use of the "latent heat of vaporization," which takes 540 calories to turn a gram of water into water vapor. This is pure heat loss. Hence sweating, even when it is "insensible" (doesn't wet the skin), is an efficient way to lose heat and one of several reasons to stay hydrated in hot weather. The problem comes when the air cannot hold any more water vapor (100% relative humidity or close to it) and this last way to lose heat is shut down. To return to the bath tub analogy, the faucet is still open (heat is coming into the body via metabolism, a warm environment and radiant heat load) but the drain is now closed. Hence the tub begins to fill with heat. If your body core temperature rises too high (say above 106 degrees), it is a medical emergency and high mortality is the rule.

So heat obviously plays a part. But it is the humidity that does the real job on most people. Air conditioning, interestingly, derives most of its effectiveness, not by dropping the temperature, but by drying out the air, allowing increased evaporative heat loss.

So now you know the real problem. No sweat.