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Temperature, Humidity, Winds, and Human Comfort

Within the human body, energy is produced by the metabolism of foods. Approximately 1800 kilocalories of energy per day is metabolized by the average person while resting, more if doing strenuous activities. Over half of this energy is converted to heat. Without some way to remove this internally-produced heat energy, the body temperature would increase indefinitely. Under certain atmospheric conditions (high temperature and high humidity), it becomes difficult for the body to remove this excess heat.

In the other extreme, problems also arise when the body loses heat too rapidly under conditions of cold temperatures and strong winds. Thermoregulation refers to the processes by which the human body regulates internal heat generation and external heat exchange so that its core temperature varies by no more than 2°C from its average of 37°C. "Core" refers to vital organs such as the brain, heart, kidneys, etc. If the body's core temperature moves outside of this range, essential life functions do not work properly.

Surprising to many, exposure to extreme heat and cold are responsible for more human deaths per year than all other weather disasters (e.g., thunderstorms, tornadoes, hurricanes, blizzards) combined.

The ideal conditions for a resting human body fall into a range called the thermalneutral zone, where the air temperature is between 20°C and 25°C, or (68-77)°F, with little wind and moderate relative humidity. Under these conditions, a resting body can easily maintain its core temperature. Outside of this narrow range, the body's thermoregulation responses take over.

Biological Control Systems

Living things, like mechanical engines, need regulators for effective operation. The regulators of living things are called biological control systems. Many homeostatic biological control systems are at work in the body. Homeostasis is the stable operation of physiological activities.

Control systems maintain a balance in the biophysical and biochemical functioning of the body. An important system is thermoregulation, which keeps the internal body temperature at a stable level in all kinds of weather.

Over half of all energy from food and other sources leaves the body as heat. Thus, the body needs a well-functioning homeostatic control system for thermal regulation.

Enzyme-controlled biochemical reactions in the body are usually most efficient at around 98°F (37°C). This temperature is the average set point for the inner body temperature of mammals. The flow diagram below shows how the body reacts to heat-related stresses.

This heat stress, or load, is a disturbance of the thermoregulatory system. The disturbance can be (a) internal temperature too high or (b) internal temperature too low. The body compensates for it in the following ways:

The human body's responses to cold core temperature

  1. Increase internal heat production by shivering (involuntary muscle contractions)
  2. Reduce heat loss by vasoconstriction (constrict blood vessels). Vasoconstriction reduces blood (and heat) flow to extremities. Thus, extremeties (forearms, lower legs) cool down. This reduces heat loss since temperature difference between extremities and outside world is less. It also reduces the surface area of warm blood that is in contact with the outside air. In severe cases this can lead to frostbite of extremities (freezing of skin).

The human body's responses to warm core temperature

  1. Sweating - body cooled by evaporation. Important to drink enough water in hot weather. This is by far the best way to lose heat. In fact, sweating is the only means by which humans can survive for long periods when the air temperature exceeds body temperature.
  2. Increase heat loss by vasodilation (widening of blood vessels). Vasodilation increases blood (and heat) flow to extremities allowing more rapid heat transfer away from body. This exposes warm blood to the outiside air over a larger surface area. NOTE: this only works if the air temperature is lower than the body temperature. If air temperature is warmer than body temperature, vasodilation would actually result in heat flow from air to blood.

These reactions are programmed by the brain's hypothalamus via information fed back from its own thermoreceptors and from thermoreceptors in the skin. Thermoregulation has a high set point, about 98°F, an indication that it is easier for the body to heat itself than to cool itself.

Thermoregulation and other forms of homeostasis do not maintain a condition at an unvarying level, but within an acceptable range. For example, most people experience a slight daily temperature variation during which body temperature dips nearly two degrees Fahrenheit (one degree Celsius) from an early evening peak to an early morning low.

The Control systems by which the brain directs the body's automatic responses to elevated or lowered core temperature are illustrated in this figure.

Hypothermia and Hyperthermia

Sometimes thermoregulatory processes fail to maintain the body's core temperature within its normal range. As the core temperature departs more and more from its optimal range, thermoregulatory processes may fail, resulting in rapidly deteriorating and potentially lethal conditions.

Hypothermia refers to conditions that develop when human core temperature drops below 35°C (95°F). Initially, shivering becomes more violent and uncontrollable. In addition, the victim has difficulty speaking and becomes apathetic and lethargic. If core temperature falls below 32°C (90°F), enzyme activity further slows. Eventually shivering ceases, muscles become rigid, and coordination deteriorates. Mental abilities also are impaired seriously and the victim is generally unable to help himself or herself. At a core temperature of 30°C (86°F), a person may drift into unconsciousness. Typically, a person becomes totally unresponsive, even to pain, at a core temperature under 26°C (79°F). Death may ensue at core temperatures below 24°C (75°F), because the heart rhythm becomes uncontrollably irregular (ventricular fibrillation) or uncontrollably halted (cardiac arrest). The lowest core temperature measured in adults who subsequently recovered from hypothermia is about 16°C (61°F).

Hypothermia rapidly can become a serious threat to survival. Only a 3°C (5.4°F) drop in core temperature greatly impairs the body's ability to regulate its core temperature. Thermoregulation essentially is ineffective when core temperature declines to 29°C (84°F). Hence, the first signs of hypothermia should never be ignored; action should be taken immediately.

Treatment of hypothermia victims takes two forms: prevention of further heat loss and addition of heat. Depending upon circumstances, further heat loss can be prevented by replacing wet clothing with dry clothing (reducing heat loss via evaporation), finding shelter from the wind (reducing heat loss via convection and evaporation), and insulating the person from the ground (reducing heat loss via conduction). The body can be heated by an external source, such as a space heater or other human bodies. If the victim is conscious, administering a warm (not hot), non-alcoholic beverage, helps warm the core from the inside. Efforts to warm the victim should never be discontinued in favor of moving the person or going for help. Medical attention, however, should be sought as soon as possible.

Hyperthermia refers to those conditions that take place when core temperature climbs to 39°C (102°F) or higher. Hyperthermia may be divided into two stages. Symptoms of the first stage, known as heat exhaustion, include profuse sweating, nausea, vomiting, and general weakness leading to an inability to continue normal activities. A person suffering from heat exhaustion should be taken immediately to a cool environment. Removal of excess clothing and sponging with water speeds the lowering of the core temperature. If necessary, evaporative and convective cooling can be accelerated by placing the victim in front of fans. The person should be given fluids orally, if tolerated, because dehydration typically is the primary cause of heat exhaustion. Most victims of heat exhaustion recover without any complications.

Failure to treat a person with heat exhaustion usually leads to a further rise in core temperature. If the core temperature reaches 41°C (106°F), enzymes begin to fail and thermoregulatory mechanisms breakdown. The victim has a rapid and strong pulse, exhibits psychotic behavior, and may slip into unconsciousness. Symptoms at a core temperature of 41°C (106°F) or higher constitute heat stroke (sometimes called sunstroke), a life-threatening emergency.

A heat stroke victim must be treated promptly because, once thermoregulation fails, core temperature rises rapidly and death may occur within a few hours. Bringing about a rapid drop in core temperature is the top priority. Although initial field treatment can be similar to that for heat exhaustion, medical attention and hospitalization should be sought as soon as possible.

Heat cramps may occur as a concurrent symptom of heat exhaustion or by themselves. Heat cramps are extremely painful contractions of the large muscles of the calf, thigh, abdomen, or shoulder and result from the excessive loss of sodium and potassium salts (electrolytes) in sweat. Heat cramps differ from exertion-induced cramps which involve the entire muscle. Rather, an individual bundle of muscles will contract for a few minutes and then relax while an adjacent bundle contracts and so on. Hence, victims of heat cramps experience the sensation of a wandering cramp throughout the entire muscle.

Victims of heat cramps should rest in a cool environment. If not nauseated, the person should be fed a commercially available electrolyte solution such as Gatorade or Powerade. If such a solution is not available, heat cramps can be relieved by administering a salt solution consisting of a one-fourth teaspoon of table salt in a liter (approximately a quart) of water. If a person complaining of severe muscle cramps is also nauseated, he or she is probably also suffering from heat exhaustion and medical attention is needed as soon as possible.

Summary of Human Responses to Thermal Stress
To Cold To Heat
Thermoregulatory responses
Constriction of skin blood vessels
Concentration of blood
Flexion to reduce exposed body surface
Increased muscle tone
Inclination to increased activity
Thermoregulatory responses
Dilation of skin blood vessels
Dilution of blood
Extension to increase exposed body surface
Decreased muscle tone
Inclination to reduced activity
Consequential disturbances
Increased urine volume
Danger of inadequate blood supply to skin of fingers, toes, and exposed parts leading to frostbite
Increased hunger
Consequential disturbances
Decreased urine volume. Thirst and dehydration.
Difficulty in maintaining blood supply to brain leading to dizziness, nausea, and heat exhaustion. Difficulty in maintaining chloride balance, leading to heat cramps.
Decreased appetite
Failure of regulation
Falling body temperature
Cessation of heartbeat and respiration
Failure of regulation
Rising body temperature
Heat regulating center impaired
Failure of nervous regulation terminating in cessation of breathing.

Influence of weather conditions on heat loss

Heat Index

In hot conditions, high humidity reduces the rate of heat loss because it slows down the rate of evaporation. Scientists have developed a variety of indices that attempt to gauge the combined effect of temperature and humidity on humans and advise people of the potential danger of heat stress. Since the summer of 1984, the National Weather Service has regularly reported the heat index (sometimes called the apparent temperature index). The heat index attempts to take into account the decreasing rate of heat loss due to the combination of high temperature and high humidity. The heat index is not a true temperature because it cannot be measured with a thermometer. The heat index is really just a number related to the computed and/or measured net rate of heat loss from the human body for a given combination of air temperature and relative humidity. The higher the heat index, the slower the rate or heat loss from the "average" human body. A heat index table is given below.

The Heat Index

Re lative Humidity (percentage)
05101520253035404550 556065707580859095100
135° 120126  
130° 117122 131 
125° 111116 123131141 
120° 107111116123130139148 
115° 105107111115120127135143151 
110° 99102105108112117123130137143150 
105° 9597100102105109113118123129135142149 
100° 919395979910110 410711011512 0126132138 144150 
95° 8788 9091939496 98101104107110114119124130136140150 
90° 838485 86878890 91 939596 98100102106 109113 117122126 131
85° 7879 808182838485868788 89909193 959799102 105108
80° 7374 7576777778 79798081 81828384 85868788 8990
75° 6969 7071727273 73747475 75767677 77787879 7980
70° 6464 6565666667 67686869 69707070 70717171 7172

   =Heatstroke risk extremely high        = Heat exhaustion possible
   =Heat exhaustion likely,  heatstroke possible        =Fatigue possible

Using the table, if the air temperature is 90°F and the relative humidity is 60%, the heat index is 100.

If the air temperature is 105°F and the relative humidity is 5%, the heat index is 97. Note that it is possible for the heat index to be lower than the air temperature if the relative humidty is low. This often happens here in the desert during the months of May and June (before the summer monsoon arrives)

INTERPRETATION: For an "average" person, the rate of heat loss will be slower when the heat index is 100 than when it is 97. This means that the human body will be more stressed (have more difficulty maintaining its core temperature) under the first set of conditions given above. The heat index is only a guide. No single chart works for all people. The degree of heat stress also depends on a person's physical fitness, body type, the ability to sweat, and other factors. Circulation of air (by fans or winds) can help because it increases the rate of net evaporation. When the heat index is forecast to reach dangerous levels, the weather service issues a heat advisory.

Finally, keep in mind that the heat index is a ficticious temperature. An object in the shade will not become warmer than the air temperature.

This following link contains U.S. Maps of Heat Index values. You can select other regions of the world using the drop down menus.

Wind Chill Index

In cold conditions, winds accelerate heat loss via forced convection, i.e., the protective millimeter layer of air is swept away, keeping the rate of heat loss high. The National Weather Service reports the windchill equivalent temperature (sometimes called the windchill factor) to alert people to the dangerous combination of cold temperatures and high winds. The windchill equivalent temperature is not a true temperature in that it cannot be measured with a thermometer. The windchill is just a number related to the rate of heat loss from the human body for a given combination of air temperature and windspeed. The lower the windchill equivalent temperature, the faster the rate of heat loss from the "average" human body. The latest windchill chart (revised in 2001) is given below.

Using the table above, if the air temperature is 10°F and the windspeed is 20 mph, the windchill equivalent temperature is -9°F.

INTERPRETATION: The rate of heat loss from exposed skin under the above conditions is the same as the rate of heat loss at an air temperature of -9°F with no wind. The windchill index will never be higher than the air temperature. Thus, an air temperature of -5°F with no wind (windchill equivalent temperature -5°F) is actually less stressful for the body compared with conditions of air temperature 10°F with a windspeed of 20 mph, since the rate of heat loss will be slower.

Note that the windchill equivallent temperature is computed for exposed skin. Clothing reduces the rate of heat loss. The best strategy is to wear several layers of loose fitting clothing. This keeps an insulating layer of air (in the spaces between clothing fibers and layers) around your body. Again, windchill equivallent temperature is just a guide, no single chart works for all people. The rate of heat loss can be much higher if the skin is wet due to additional evaporative cooling.

The windchill equivallent temperature is a ficticious temperature. Dry objects (fingers for example) will not become colder than the air temerature. However, high winds, in below-freezing air, can remove heat from exposed skin so quickly that the skin may actually freeze and discolor. The freezing of skin, called frostbite, usually occurs on the body extremities first because bloodflow to these areas is reduced to conserve body heat for vital internal organs.

The following link contains US Maps of windchill equivalent temperature values. You can select other regions of the world using the drop down menus.

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