The absence or presence of heat is one of the primary environmental factors affecting human comfort. We pay enormous attention to the five main sensory organs in our bodies, yet often overlook our true sixth sense, thermal sensitivity.

Figure 1 - Cooling off...

Thermal sensation results from two sets of sensory organs within the skin.

• The first of these, the Bulbs of Krause, are sensitive to heat loss. They number around 150,000 and lie within 0.5mm of the surface of the skin. Whilst spread throughout the body near the openings to sweat glands, there is some increased concentration around the fingertips, nose and bends of the elbow.
• The Organs of Ruffini, however, are sensitive to heat gain and number only around 16,000. These lie much deeper within the skin, mostly around the lips, nose, chin, chest, forehead and fingers. Due to the increased insulation provided by skin depth, these are much slower to react to changing environmental temperature than the bulbs of Krause. These two sets of organs allow us to sense our thermal surroundings, allowing us to determine if we are gaining or losing heat.

As warm-blooded mammals, humans produce energy by metabolising food, with most of this energy taking the form of heat. This metabolic heat is produced by the body all the time, mainly as a result of muscular activity, although almost all bodily functions produce some heat. In general the more active we are, the more heat we produce.

Figure 2 - Physical activities with different metabolic rates.

Heat is transported around the body by the blood. To balance the metabolic input, heat is continually lost to the environment through the skin and through the surfaces of the lungs. For the proper functioning of the organs of the body, and particularly for that of the brain, the temperature of the internal organs (the deep-body or core temperature)

must be maintained constant, at around 37°C. If it rises above 41°C or falls below 30°C, death is imminent. This does not leave much margin for error, so the body's ability to adjust to its thermal environment needs to be quite sophisticated.

The controlling mechanism for this is the brain. It constantly references feedback from its external heat sensing organs against its own internal 'thermometer' in the hypothalamus. If brain temperature falls outside these very close limits, the body will react physiologically to restore heat balance.

Extreme Heat

Extreme heat in a desert environment

If, due to an increased environmental temperature or metabolic rate, the body begins to heat up, a process called vasodilation is initiated in the hypothalamus. This involves the dilation of blood vessels near to the surface of the skin to increase its temperature. This leads to increased radiant and convective heat loss.

If the body temperature is still increasing, the sweat glands are activated to produce moisture on the skin surface. When this moisture evaporates, latent heat of vaporisation is drawn from both the surrounding environment and the skin. As sweat flows directly onto the surface of the skin, conduction from circulating blood immediately beneath forms one of the most significant components. Sweat can be produced at up to four litres per hour for short periods and is supplemented by the evaporation of moisture from within the lungs and respiratory tracts.

If heat build up continues, hyperthermia sets in and heat stroke may develop. This occurs when deep-body temperature reaches about 40°C and can cause fatigue, headache, nausea, shortness of breath and, in some cases, mental disturbances such as apathy, poor judgement and irritability.

After heat stroke is developed, sweating stops, rapidly increasing body temperature until serious brain damage results in coma and imminent death.

Extreme Cold

Extreme cold in an arctic environment

In cold conditions the body's first response to heat loss is vasoconstriction. This involves the contraction of blood vessels near to the surface of the skin, increasing its insulative value and reducing heat loss. If heat loss continues, blood supply to extremities such as fingers and toes may be cut off completely before deep-body temperature is affected. If these extremities subsequently freeze, this is known as frostbite and often results in their loss.

Vasoconstriction is often accompanied by the erection of hair follicles (also known as goose pimples or pilo-erection) which, if enough matted hair were present, would greatly increase its insulation potential. With even greater heat loss, shivering is initiated. This is simply involuntary spasms within certain muscle groups designed to increase metabolic heat production.

When deep-body temperature falls below 35°C, hypothermia sets in. Metabolic controls cease to function and body cooling is increased. In some extreme cases, where body cooling occurred very quickly, individuals have survived long periods of almost suspended animation. If body cooling occurs at slower rates, death results at a temperature of between 25 and 30°C.

Related Links

CBD-102. Thermal Environment and Human Comfort

http://www.nrc.ca/irc/cbd/cbd102e.html

Vegetation Influences on Human Thermal Comfort in Outdoor Spaces

http://www-fa.upc.es/personals/jroset/lyonvege.html

FIRST AID - Skills for Life

http://firstaid.eire.org/Firstaid_index.html

Hypothermia Prevention, Recognition and Treatment

http://www.hypothermia.org/jama.htm

HYPOTHERMIA

Surviving the Cold: http://www.worksafebc.com/pubs/brochures/howto/hypothrm.asp

Hot and Humid Weather Increases Risk for Hyperthermia

http://www.uchc.edu/ocomm/newsreleases01/july01/hyperthermia.html

Hypothermia and Hyperthermia in the Elderly

http://www.rnceus.com/hypo/hypoframe.html