Bodily mechanisms of heat transfer and thermostatic control
Heat is exchanged between the body and its environment by four modes: evaporation (E), radiation (R), convection (C) and, to an insignificant extent because of the small area of contact usually involved, conduction. There is also a small loss by the rejection of excreta but this is generally ignored. A heat balance equation can be written if we include an additional element, S, representing the positive or negative storage of heat in the body that would cause the deep tissue temperature to rise or fall:
M-W=E + R + C + S (4.1)
For a normally clothed, healthy human being in a comfortable environment and engaged in a non-strenuous activity, S is zero and the thermo-regulatory system of the body is able to modify the losses by radiation and convection to maintain a stable, satisfactory temperature. Evaporative losses occur in three ways: by the exhalation of saturated water vapour from the lungs, by a continual normal process of insensible perspiration, and by an emergency mechanism of sweating. Insensible perspiration results from body fluids oozing through the skin under osmotic pressure according to Whitehouse et «/.(1932) and forming microscopic droplets on the surface which, because of their small size, evaporate virtually instantaneously, not being felt or seen and hence termed insensible. Sweating is entirely different and in comfortable conditions should not occur: if body temperature tends to rise the thermoregulatory system increases the evaporative loss by operating sweat glands selectively and flooding strategic surfaces. In extreme cases the body is entirely covered with sweat that must evaporate on the skin to give a cooling effect—if it rolls off or is absorbed by clothing its cooling influence will be nullified or much reduced.
Evaporative loss is a function of the difference in vapour pressure between the water on the skin and that of the ambient air. It also depends on the relative velocity of airflow over the wet surface. The two following equations are sometimes useful to solve numerical problems although equation (4.2), for parallel airflow over a lake surface (or the like) is considered to give an underestimate. Equation (4.3) describes the case of transverse airflow, as across a wet-bulb thermometer.
Evaporative loss (W m“2) = (0.0885 + 0.0779v)(pw — ps) (4.2)
Evaporative loss (W m~2) = (0.018 73 + 0.1614v)(pw — ps) (4.3)
Where the relative air velocity, v, is in m s-1 and pw and ps, the vapour pressures of the water and the ambient air respectively, are in Pa.
Losses occur by radiation if the skin temperature exceeds that of the surrounding surfaces and by convection if it is greater than the ambient dry-bulb. The average temperature of the surrounding surfaces is termed the mean radiant temperature, Trm, and is defined as: the
Surface temperature of that sphere which, if it surrounded the point in question, would
Radiate to it the same quantity of heat as the room surfaces around the point actually do. The mean radiant temperature thus varies from place to place throughout the room. Bodily surface temperature is influenced by the type of clothing worn, its extent, the activity of the individual, the performance of the thermo-regulatory system and the rate of heat loss to the environment. Signals are sent by nerve impulses from the brain in response to changes in blood temperature to regulate the flow of heat from the warmer, deep tissues of the body to the cooler, surface tissues. This regulation is done two ways: by changing the rate of sweat production and by dilating or constricting the blood vessels (the vascular system) beneath the skin. Vaso-dilation increases the flow of blood to the surface and so the flow of heat from the deeper tissues. Conversely, vaso-constriction reduces the flow of blood, the skin temperature and the bodily heat loss. A skin temperature exceeding 45°C or less than 18°C triggers a response of pain according to ASHRAE (1997) and subjective sensations for mean skin temperatures of sedentary workers are: 33.3°C comfortable, 31°C uncomfortably cold, 30°C shivering cold and 29°C extremely cold. For a more strenuous activity such temperatures might be reported as comfortable. A skin temperature of 20°C on the hand may be considered uncomfortably cold, one of 15°C extremely cold and 5°C painful. Higher ambient air temperatures than the skin temperatures mentioned can be borne because of the insulating effect of the air surrounding the surfaces of the body and some tolerances quoted given by ASHRAE (1989) are: 50 minutes at 82°C, 33 minutes at 93°C and 24 minutes at 115°C, for lightly clad persons in surroundings with dew points less than 30°C. Tolerance decreases rapidly as the dew point approaches 36°C. A rise in body temperature of a few degrees, because of ill-health or inadequate heat loss, is serious with possibly fatal results at above 46°C when the thermo-regulation control centre in the brain may be irreversibly damaged: sweating can cease and vaso-constriction become uncontrolled with the onset of heat production by shivering. On the other hand, a fall in the deep temperature of the body to below 35 °C can also cause a loss of control by the thermo-regulation system and although recovery from a temperature as low as 18°C has occurred, according to ASHRAE (1989), 28°C is taken as the lower survival limit. The normal response of the body to a fall in temperature after vaso-constriction is the generation of heat by muscular tension and, subsequently, by involuntary work or shivering.
According to Gagge et al. (1938) experimental evidence shows that unclothed and lightly clad people can be comfortable at operative temperatures (see section 4.8) of 30°C and 27°C, respectively. These temperatures are the mid-points of narrow ranges (29°C to 31°C and 25°C to 29°C) within which there is no change of evaporative loss and no body cooling or heating, the deep tissue temperature being maintained at a constant value without physiological effort. Above and below these ranges are zones of vaso-motor regulation against cold and heat wherein the control system of the body can keep the deep tissue temperature constant (albeit with some change in skin temperature) by vaso-constriction and dilation, respectively. Below the cold zone muscular tension etc. takes over until eventually deep temperature can only be maintained at a satisfactory level by putting on more clothing. At the upper end of the range of operative temperatures the zone of control over heat is much smaller and vaso-dilation only suffices until the skin temperature approaches to within 1 °C of the deep body temperature, after which the sweat glands must work if the thermal balance between the person and the environment is to be maintained.
It follows from the foregoing that skin temperature and evaporative cooling from the skin will have a significant influence on comfort. Belding and Hatch (1955) defined a heat stress index as the ratio of the total evaporation loss in bodily thermal equilibrium to the maximum loss by evaporation if the skin were entirely wetted by regulatory sweating, is sometimes used. The ASHRAE scale of effective temperature (see section 4.8) makes use of the concept of heat stress.
Posted in Engineering Fifth Edition