A record-dissolving heat wave continues its sweep across the country this week, sending anyone concerned about climate change into literal and figurative sweats.

At least 52 deaths have been blamed on the heat. Temperatures in New York climbed to 99 degrees Fahrenheit last weekend. If the human body is about 98.6 degrees, why are equivalent outdoor temperatures so uncomfortable, even dangerous?

Because our bodies need to disperse heat, and they can't do that effectively when the air temperature is close to our body temperature. Our muscles and metabolism generate heat continuously. We transfer that heat into our surroundings by sweating, exhaling warm air and circulating blood near the surface of our skin to cool. When the temperature gradient (or difference) between the body and the air is high, heat flows easily from us into the environment, and we cool down. But when the weather hovers around our internal temperature, our inner swelter lingers, and we feel hot and uncomfortable. Humidity makes things worse by interfering with the vaporization of sweat, one of the human body's main cooling mechanisms.

By the same token, frigid weather draws the heat from our bodies faster than we can produce it, and our core temperature falls. A person's thermal comfort, or satisfaction with the temperature of the environment, depends on factors as varied as metabolic rate, body fat and age. For instance, those with heavier builds have a lower ratio of skin surface area to mass — they evaporate heat less efficiently than the small-framed. And fat absorbs warmth readily, making the obese more susceptible to heat stress. Other groups shown to be especially sensitive to hot weather include pregnant women, the disabled, and people younger than 14 or older than 60. (The body's effectiveness at thermoregulation declines with age.)

All else being equal, the human body operates most efficiently when the air temperature is about 70 degrees. In the late 1950s, a U.S. Marine Corps outfit in South Carolina attempted to factor out some of this "all else" by creating the wet bulb globe temperature index. The measure takes into account temperature, humidity, wind speed, sun angle and cloud cover. Safety advisories pinpoint a WBGT value of 77 degrees Fahrenheit under normal humidity and other factors as an "outer limit" for humans: Exert yourself under much hotter, damper, or more stagnant conditions, and you run the risk of heat stroke.

Because exercise causes the body to generate so much extra heat, optimal temperatures for intense physical activity are lower than those for daily life. Athletes can raise their core temperatures six degrees just by working out. Add an environment that makes heat dispersal more difficult — not to mention possible dehydration from sweat losses that sometimes exceed six liters (for marathoners) or two liters per hour (team game players) — and performance can take a nosedive. (Might the misery of exercise in torrid weather explain why the South boasts higher obesity rates than cool and crisp Colorado?)

Endurance can also diminish in the heat as the heart works ever harder to power the same feats. For example, researchers in Darwin, Australia, observing a long-distance runner taking a 30-minute jog through the humid air, noted that his body temperature increased from 98.96 degrees to 105.8 degrees. When he'd gone on a similar jaunt under cooler conditions, his temperature had risen by just two degrees. Such a spike spells trouble for maintaining an optimal heart rate: The man's rate soared to 200 beats per minute during the last 15 minutes of his run, where, previously, it was a more sustainable 154 beats per minute.

There are some benefits to exercising in the heat, however. It may enhance later athletic performance in more temperate weather.

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