Limits to Survivability in Extreme Heat

In the United States, the Heat Index is the most commonly used measure of the physiological response to combinations of temperature and humidity. Values of the Heat Index above roughly 105 and 130 degrees Fahrenheit are referred to as "dangerous" and "extremely dangerous", respectively, but without regard for length of exposure, degree of shading, level of exertion, or age. Here, we introduce a simple prognostic model of human core temperature that allows for a quantification of these various influences. Designed to model the response to extreme heat, many of the uncertain empirical relations used in other thermoregulation models -- such as the blood flow rate and sweat rate -- drop out of the equations. It is shown that the Heat Index can be mapped more usefully onto a maximum survivability time, which depends on the person and circumstance, including body size, skin albedo, solar exposure, and level of exertion.