Air temperature alone is not an adequate indication of environmental warmth. Wind, sunshine and humidity are also important. The notion that all these factors might be combined into a single figure indicating warmth is attractive. In this 1972 monograph, the problem of constructing such an index of heat stress is examined from a theoretical point of view. The approach involves a step-by-step examination of the physical principles which govern heat exchange at the skin surface. Mathematical terms are carefully defined and the physical concepts explained and illustrated. A great deal of information has been culled form specialist reports and is brought together and assessed critically. Although it is an older work, this book will continue to be of interest to all concerned with advancements in thermal physiology.