Heat Stress
Authors: Adams Thomas 
Published: August 2005

This chapter's goal is to provide useful, interesting, and up‐to‐date information about the way people respond to acute and chronic heat stress. It also presents some groundwork about biological control systems, how heat is produced and stored in the body, and how it is exchanged with the environment. This background is necessary to understand the many complex and interactive phenomena associated with heat stress and with the challenges of heat strain.

It is important to recognize the uniqueness of heat as a form of energy to understand issues of heat stress and strain and to see its special role in human welfare. No other potentially endangering energy, for example, is produced by the body itself, none other is so essential for life, and none other is the thermodynamic destiny in entropy for all other forms of energy.

No form of energy other than heat so directly affects life processes. Humans normally function in only a narrow range of deep body temperature. It is not always easy to spot the sources of heat danger or to gauge their effects. Heat‐induced disabilities come from net heat gain from the environment and also from that produced by metabolic activity, which alone can be crippling and lethal. Danger levels for most energy and molecular environmental stressors are predictable for everyone, but they are not for heat. It is not hard, for example, to define a “threshold limit value” and safe exposure levels for gamma and ultraviolet radiations, sound intensity, and exposures to concentrations of specific chemicals. A toxic level for one person is just about as endangering as it is for someone else. It is quite different for heat.

The effects of heat stress must be seen in a different way. One level of environmental heat exposure and rate of heat production from work or exercise may be comfortably, safely and easily tolerated by one person, yet be uncomfortable, disabling, and potentially lethal for another, even though they work side by side at comparable tasks. Solving the complex problems of human thermal safety requires more than just measuring the intensity of contributing environmental factors. It requires additional analyses of a person's internal and external thermal environments and body heat distribution and also of many unique personal characteristics

Situations that challenge human thermoregulation must be surveyed at two levels. One is a need to look at the conditions of heat stress, the other must evaluate the level of heat strain. Heat stress and strain are functionally related, of course, but one does not predict the other.

Distinguishing between factors of heat stress and human heat strain is essential. It is naive, dangerous, and potentially lethal to consider that measuring only factors of heat stress, no matter how completely and precisely, or using one's own thermal comfort is any reliable gauge for another person's level of heat strain (10–12). Evaluating human heat strain is a complex but interesting detective story. Lessons learned are applicable to issues of the workplace, and they are also pertinent to each of us personally, to our family members, and to all life forms on the planet.