Hypothermia [body core temperature belowF] occurs when heat loss is greater than metabolic heat production. Early signs and symptoms of hypothermia include shivering, euphoria, confusion, and behavior similar to intoxication. Lethargy, muscular weakness, disorientation, hallucinations, depression, or combative behavior may occur as core temperature continues to fall. If body core temperature falls below .C F, shivering may stop and the patient will become progressively delirious, uncoordinated, and eventually comatose if treatment is not provided.
During cool or cold weather marathons, the most common illnesses are hypothermia, exhaustion, and dehydration. The most common medical complaints are weakness, shivering, lethargy, slurred speech, dizziness, diarrhea, and thirst. Runner complaints of feeling hot or cold do not always agree with changes in rectal temperature. Dehydration is common in cool weather. Runners should attempt to replace fluids at a rate that matches their sweat and urine losses. Cases of hypothermia also occur in spring and fall because weather conditions change rapidly and runners wear inappropriate clothing that becomes sweat-soaked during training or competition.
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Hypothermia may occur during races, for example when distance runners complete the second half of the event more slowly than the first half. Evaporative and radiative cooling increase because wet skin from sweat, rain, or snow and clothing are exposed to higher wind speed at a time when metabolic heat production decreases. Hypothermia also occurs after a race, when the temperature gradient between the body surface and the environment is high. Subfreezing ambient temperatures need not be present and hypothermia may develop even when the air temperature is -C -F. A WBGT meter can be used to evaluate the risk of hypothermia see Postscript: Measurement of Environmental Stress, pageCold wind increases heat loss in proportion to wind speed; windchill factor. The relative degree of danger can be assessed see figureon pageWind speed can be estimated; if you feel the wind in your face the speed is at least km h- kph [ mi h- mph]; if small tree branches move or if snow and dust are raised, approximately kph mph; if large tree branches move, kph mph; if an entire tree bends, about kph mph.
To reduce heat loss, runners should protect themselves from moisture, wind, and cold air by wearing several layers of light, loose clothing that insulate the skin with trapped air. An outer garment that is windproof, allows moisture to escape, and provides rain protection is useful. Lightweight nylon parkas may not offer thermal insulation but offer significant protection against severe windchill, especially if a hood is provided. Wool and polyester fabrics retain some protective value when wet; cotton and goose down do not. Areas of the body that lose large amounts of heat head, neck, legs, hands should be covered.
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