Examination of Use of Heat-Exchange Equations for Determining Changes in Body Temperature

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Abstract
When acclimatized laborers work continuously for 4 hrs. they reach the same equilibrium level of body temp. as at similar rates of work towards the end of a spell of training in mild conditions. These levels are 99.4-99-6[degree]F up to 90[degree]F air temp. (with 100% humidity) during light work; 99.7-100.0[degree] up to 87[degree] air temp. during moderate work; and 100.0-100.4[degree] up to 84[degree] during hard work. Wind-velocity alterations from 50-400 ft./min. and changes in air temp. have no effect on the level of body temp. in these ranges of air temp. but the level is closely dependent upon the rate of work. The time taken to reach equilibrium is 1 hr. Above these air temps., new and higher equilibrium levels of body temp. are reached, the time taken increases, and wind-velocity now exerts an effect. When these body temps. are compared with those detd. from available heat exchange equations for the same heat stress conditions, 3 important points of difference emerge. Firstly, there is a range of air temp. in which exptl. body temps. reach similar equilibrium levels, irrespective of wind-velocity, whereas at only one air temp., at each wind-velocity, is the theoretical body temp. at this equilibrium level. Secondly, above a critical air temp. (for example, 90[degree] F during light work) body temp. rises to new equilibrium levels on a rapidly rising curve. Theoretical equilibrium levels increase linearly in relation to air temp. Thirdly, wind-velocity exerts a significant effect only where the curve of body temp. rises sharply, whereas wind-velocity has a uniform influence on theoretical body temps. over the whole range of air temps. These differences can be explained in terms of a physical model of heat transfer in man which incorporates 2 separate heat capacities, representing deep and superficial tissues, linked by a variable heat resistance. The superficial heat capacity is linked with the environment by means of a fixed heat resistance, representing heat exchange by radiation and convection, and a variable heat resistance, representing heat exchange by evaporation.

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