Module 1 - situation: body ambient bondgraph model using heat flux  (Page 2/3)

Focusing this paper upon conditions where the body is at risk to over heating, we can describe three primary mechanisms of heat transfer: radiation, convection and conduction, and evaporation.

Heat will radiate to or from the body based on the temperature differential between the skin and external objects. Air temperature contributes only a small factor in heat radiation, as air is a poor radiator. If the body is in direct or reflected path of the sun’s rays, the body will absorb heat.

Heat will transfer to or from an object in contact with the skin. In the case where the object is air, this is known as convection, be it natural (still air) or forced (moving air). In the case the object is something other than air, it is conductive heat transfer. Conductive heat transfer is not typical of most environments where heat illness is a risk.

In evaporation, sweat is diffused across the skin and condenses on of the skin. Heat is transferred from the skin to ambient as the sweat changes phase from liquid to gas. This is the most effective heat transfer mechanism of the human body. In environments where the relative humidity is high, sweat will not evaporate, this mechanism provides no relief. An average person will not tolerate temperatures above 33ºC, even at rest, without the ability to dissipate heat through evaporation.

Figure 1: Modes of Heat Generation and Transfer for the Human Body

The primary response of the body to an increased core temperature is to increase blood flow to extremities by dilating the capillaries of the blood system, effectively using parts of the body as a thermal sink. Mass flow can further be improved by increasing the heart rate. As sweat is secreted to the skin, the sweat evaporates and energy is further dissipated through phase change.

Heat stress and risks

Heat stress illness effects can be either acute, such as heat stroke, heat exhaustion, heat cramps, fainting, and decline of performance; or chronic, such as loss of ability to tolerate heat, hypertension, heart muscle damage, reduced libido and impotence.

Figure 2: Spectrum of Heat Stress Illness

Description of occupational and leisure tasks

Aerobic athletes, such as the example of a runner during a marathon, will generate heat in a consistent manner for an extended period of time. The environment can be variably providing either beneficial or antagonistic conditions. Clothing and equipment do not generally impede the dissipation of heat.

Anaerobic athletes, such as the case of a football player, generate heat in short durations. The effects of this heat generation can be cumulative if periods of rest are not long enough to decrease body temperature, or if periods of exercise are long enough to exhaust reserves of body fluids. Environmental conditions are variably beneficial or antagonistic. Clothing and equipment can impede heat dissipation to the environment.

Some occupational tasks generate heat in short durations, but over extended periods of work. Examples include roofers, construction workers, or farmers. Antagonistic conditions are common. Clothing and safety equipment generally provide some impediment to heat dissipation.