What we think of as normal body temperature is an equilibrium core temperature achieved in conditions of thermal neutrality. A constant body temperature in itself does not mean you are thermally neutral. Constant body temperature has been recorded in a man running at a marathon pace in extremely hot but dry air, as illustrated in the panel on the lower left. His temperature was perfectly normal but this is not what we mean by normal temperature.

Our concept of normal temperature and thermal neutrality implicitly is limited to environments in which a non-exercising person can achieve an equilibrium core temperature without resorting to shivering or sweating.

The range of environments that are thermally neutral is wide. I have illustrated one extreme in the panel with the cactus and the umbrella. Body core temperature stabilizes in this nearly naked person without the necessity of sweating.

At the other extreme, the person next to the igloo is thermally neutral thanks to the heavy clothing that maintains a local environment next to his skin similar to the one enjoyed by the person under the beach umbrella.

We would expect to measure a normal core temperature in both of these individuals and we would expect core temperature to remain constant as long as their thermal conditions and metabolic rate do not change except -- after a while -- we might expect changes with the circadian rhythm or if they dropped off to sleep. .

This extreme range of air temperatures compatible with thermal neutrality contrasts with a narrow range of skin temperatures. In terms of skin temperature, the neutral zone is something like 33 to 35 degrees, based on physical measurements, that is to say, analyses of thermal balance. It turns out that the range of thermal neutrality in the subjective sense -- what people will tell you feels thermally neutral -- corresponds to very nearly the same range of skin temperatures.

Click for next page , on Experimental origins of the neutral zone concept

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