Health risks during exercise in the cold

If humans had retained the ability of lower animals, such as reptiles, to tolerate low body temperatures, we could survive extreme hypothermia. Unfortunately, the evolution of thermoregulation in humans has been accompanied by a diminished ability of tissues to function effectively with increases or decreases in temperature. This section deals briefly with those health risks associated with cold stress, including hypothermia and frostbite. The American College of Sports Medicine published a comprehensive position stand, “ Prevention of Cold Injuries during Exercise”, in 2006 that addresses these topics in much greater detail.

Hypothermia

Individuals immersed in near-freezing water will die within a few minutes when their rectal temperature decreases from a normal level of 37°C(98.6°F) to 24 or 25°C(75.2 or 77°F). Cases of accidental hypothermia, as well as data obtained from surgical patients who are intentionally made hypothermic, reveal that the lethal lower limit of body temperature is usually between 23 and 25°C(73.4-77°F), although patients have recovered after having rectal temperatures below 18°C(64.4°F).

Once core temperature falls below about 34.5°C(94.1°F), the hypothalamus begins to lose its ability to regulate body temperature. This ability is lost when the internal temperature decreases to about 29.5°C(85.1°F). This loss of function is associated with slowing of metabolic reactions to one-half their normal rates for each 10°C(18°F) decline in cellular temperature. As a result, low core temperature can cause drowsiness and even coma. 

Cardiorespiratory effects

The hazards of excessive cold exposure include potential injury to both peripheral tissues and the cardiovascular and respiratory systems. Death from hypothermia has resulted from cardiac arrest while respiration was still functional. Cooling primarily influences the sinoatrial node, the heart’s pacemaker, leading to a gradual decrease in heart rate, and, ultimatively, cardiac arrest.

People have questioned whether rapid, deep breathing of cold air can damage or freeze the respiratory tract. In fact, the cold air that passes into the mouth and trachea is rapidly warmed, even when the temperature of inhaled air is less than -25°C(-13°F). Even at this temperature, when a person is at rest and breathing primarily through his or her nose, the air is warmed to about 15°C(59°F) by the time it has traveled about 5cm(2 in.) into the nasal passage. As shown in the figure below, extremely cold air entering the nose is quite warm by the time it reaches the back of the nasal passage, thereby posing no threat of damage to the throat, trachea, or lungs. Mouth breathing, which often occurs during exercise, may result in cold irritation to the mouth, pharynx, trachea, and even bronchi when the air temperature is below -12°C(10°F). Excessive cold exposure also affects respiratory function by decreasing respiratory rate and volume.

Treatment for hypothermia

Mild hypothermia can be treated by protecting the affected person from the cold and providing dry clothing and warm beverages. Moderate to severe cases of hypothermia require gentle handling to avoid intiating a cardiac arrhythmia. This requires slowly rewarming the victim. Severe cases of hypothermia require hospital facilities and medical care. Recommendations to prevent cold exposure-related injuries were outlined by the American College of Sports Medicine in its 2006 position stand on cold illness during exercise. 

Frostbite

Exposed skin can freeze when its temperature is lowered just a few degrees below the freezing point(0°C,32°F). Because of the warming influence of circulation and metabolic heat production, the environmental air temperature(including windchill) required to freeze exposed fingers; nose and ears is about -29°C(-20°F). Peripheral vasoconstriction helps the body retain heat. Unfortunately, during exposure to extreme cold, the circulation in the skin can decrease to the point that the tissue dies from lack of oxygen and nutrients. This is commonly called frostbite. If not treated early, frostbite injuries can be serious, leading to gangrene and loss of tissue. Frostbitten parts should be left untreated until they can be thawed, preferably in a hospital, without risk of refreezing.