Throughout history, sailors and soldiers, pioneers and prospectors, traders and trappers have traveled, worked and fought in all climates. Men and women of every race and all ages have explored for the sake of adventure or personal gain; some braved the elements in search of peace of mind, and others sought respite from persecution. Not all survived.
Colonel James M. Adam was a Consultant Physiologist with the British Army for nearly thirty years, responsible mainly for problems of maintaining combat effectiveness of British soldiers in all extremes. He served in Antarctica, Korea, Malaysia, and the deserts of the Middle East, and left the Army to help set up the Institute of Environmental and Offshore Medicine, Faculty of Medicine, University of Aberdeen.
It is perhaps a sad reflection on human nature how often advances in the technology of living and surviving have followed fast on the heels of warfare: and this applies particularly to research into the problems of heat and cold. Cyrus the Great, for example, was worried about the possibility of heat illness in his troops when he was reorganizing the armies of the Medes and Persians in 539 bc—so he ordained that they are taken into the desert every day to work until they sweated. In a similar vein, the potentially lethal effects of cold-water immersion seem to have first been noted during the naval wars between the Greeks and Persians in the fifth century BC. Such lessons have had to be relearned again and again up to the intensive research that followed World War II.
The ease of high-speed air travel to distant climes, whether for business or leisure, has occasioned risks to many travelers. The fatigue of such flights, especially those with a large easterly or westerly component (causing the greatest ‘jet lag’) will upset particularly the unfit person, the ill, the infirm, and the aging. On arrival, a hostile climatic environment may compound the stress.
Some general considerations
Physicists have long since determined the rules that govern the exchange of heat from one object to another; the fundamental principles of such heat exchanges are of great relevance to an understanding of the effects on the human body of climatic extremes.
The net flow of heat energy is always from hotter to colder objects. Such heat flow can occur in three main ways:
Radiation All objects radiate heat energy in the form of electromagnetic waves of various wavelengths; the hotter an object, the more heat it radiates. Radiant energy can be absorbed by another, distant object, the amount absorbed depending on such factors as the color and texture of the receptive object’s surface (light-colored surfaces tend to absorb less than dark-colored surfaces). Heat can pass by radiation even across a vacuum, as when the sun heats the earth, across space.
Convection is the term given to the transport of heat by the motion of the warmed fluid (or gases). A small amount of the fluid is in contact with the heat source and is heated by conduction (see below). As its temperature rises the fluid expands, becomes lighter and so it rises away from the heat source. Further fluid replaces that which has floated away, is heated in turn, and so on. As the heated fluid cools elsewhere it contracts, becomes denser, and sinks. In this way, a convection current is set up.
Conduction is the transfer of heat from a heat source to a cooler object by direct contact and without demonstrable motion of the parts of the object such as described above in ‘convection current’. Metals conduct heat away most readily, while still air is the most effective barrier to heat lost by this route.
Body temperature and its control
The upper limit of the average normal temperature of the human body at rest is 36-9°C (98-4°F), although patients inactive in bed may register a normal value of 36-4°C (97*5°F). In very hot or very cold climates accurate temperature measurements cannot be made by means of a thermometer in the mouth, and instead, the thermometer must be placed in the rectum.
The human body exchanges heat with its surroundings by the avenues mentioned above. Heat is gained by the body by the absorption of radiation from distant hot objects (such as the sun)
and also by conduction and convection from the surrounding air or water if these are at a higher temperature than the body. Conversely, heat can be lost by radiation to distant cooler objects and also by conduction and convection to the surrounding air or water if these are at a lower temperature than the body.
In addition, two other processes have an important influence on the body’s heat balance. First, the body is always gaining heat produced during the digestion of food and from working muscles. Second, heat is lost by the evaporation of sweat from the surface of the skin. Heat loss by sweat evaporation is at the rate of 1464 kilojoules per liter of sweat (350 kilocalories per pint), which is a considerable but at times very necessary heat loss. In experiments with fit paratroopers working under severe desert conditions, I have measured sweat losses of more than 10 liters (17.5 pints) daily providing a heat loss of about 25 000 kilojoules (6000 kilocalories) just to keep body temperature constant.
Our bodies function efficiently by means of a myriad of chemical changes that occur almost simultaneously. These have an optimum temperature reaction for the continued health of 36-9°C (98-4°F), with the slight variations described above.
Many of the vast numbers of reactions are centered in the main organs of the body, and of these, the brain is the most sensitive to temperature change. Indeed, a departure of just over 0-5°C above or below the normal 36-9°C central body (core) temperature, as measured in the rectum, can be demonstrated to produce malfunction of this organ and of the parts of the body that it governs. The further the departure from normal, the greater the degree of malfunction, so that a rise of a core temperature of 2°C to 38-9°C (102*0°F) is accompanied by signs and symptoms of definite illness. Equally, a fall of the same magnitude to 34-9°C (94-8°F) takes the person into the realms of hypothermia, which is defined as a core temperature of 35-0°C (95 0°F) or less.
All evidence available points to the existence of a heat-regulating center in the hypothalamus (an important area of the brain). The control mechanism is influenced not only by the temperature of the blood reaching the brain, but also be nerve impulses arising in the skin. The mechanisms by which the hypothalamus controls the body temperature are very complex and cannot be considered in detail here. Suffice it to state that in the fineness and speed of its control, the hypothalamus is a better thermostat than most of those available commercially.
Categories of climatic extreme
A better understanding of the possible ills which may result from climatic variations requires a redefinition of some basic variable factors. Consideration of the extremes of temperature and humidity give four main climatic categories:
Hot!wet describes the environment of the rainforest, the jungle and secondary jungle in which the air or shade temperature is rarely above 38-0°C (100°F) in the daytime and is more commonly 33-34°C. The combination of abundant moisture, tree canopy, and frequent cloud cover serves to maintain the temperature at a fairly constant level all the year round, with little variation between day and night. The humidity is high, varying between about 65 percent by day and 100 percent by night; airspeeds are generally low.
Holiday describes the climate of the tropical and subtropical desert and semi-arid tracts of land. The tropical deserts have low humidity, scarce or absent vegetation, cloudless skies, intense sunshine, airspeeds varying from low to violent with dust and sand-storms, and scanty and erratic rainfall. At night, the clear skies permit rapid heat loss to space by radiation and convection, so there may be heavy dew and occasional frost. There is thus a high day-to-night variation in air (shade) temperature from as high as 55°C by day (the record is 58*05°C) to as low as -5-6°C in the Western Sahara or — 42°C in the winter-time Gobi desert at night.
Cold/wet describes the climate of large areas of the world, including most of western Europe north of the latitude of the Pyrenees. The term ‘temperate zone’ is often applied to such regions, but is something of a misnomer since it may lead people to underestimate the speed with which changes in the climate can occur, as well as its hazards and severity. The air temperature ranges between about 10°C and —2°C (50°F and 28°F), with rain, hail, sleet, mud, puddles, and high winds as possible accompaniments.
Cold/dry is the environment in which the air (shade) temperature rarely if ever rises above the freezing point of water (0*0°C or 32°F) at any time of the day. The terrain is frozen, and there is no free still water (as in puddles and ponds). Snow and ice cover may be total, skies are often clear of clouds, and sunshine may be brilliant, especially in the absence of wind. However, movement and navigation can sometimes be hampered by ‘white-out’ conditions, snow precipitation, and blizzards.