It's not just the heat; it's the humidity, in this Moment of Science.

Our bodies generate heat. The more active we are, the more heat we generate.

To keep our inner body temperature at the level where it belongs, we must dissipate that heat. If we don't, our body temperature begins to rise. We face heat stroke at a body temperature of about 106 degrees Fahrenheit, and death at a body temperature of about 110.

Heat flows from warmer material into cooler material. When the surrounding air is cooler than our body temperature, our skin is cooler than the inner body. That makes heat flow out, as it should.

If the air temperature gets close to our body temperature, something more is needed to keep the skin cooler than the inner body. We sweat.

When we sweat, energy represented by heat in our skin drives water molecules apart and sends them off into the air. In other words, evaporation cools our skin; the all-important temperature difference between inside and outside is restored, and heat continues to flow outward, even if the air temperature is close to our body temperature.

We're in trouble if the air is not only very hot but humid. If the air is nearly saturated with water vapor, then evaporation effectively stops and sweating no longer cools our skin. Heat no longer flows out of our bodies, so our body temperature rises, especially if we continue to generate a lot of internal heat through hard work.

Heavy exertion, high temperature, and high relative humidity is a dangerous combination that no one can endure for long.