Rationale:

• Hypothermia occurs when persons in a cold environment cannot maintain normal body temperature. The body responds to cold by increasing heat production and maintaining core temperature through peripheral vasoconstriction and shivering. The associated increase in cardiac output increases urine production and, thus, volume loss, predisposing the person to dehydration.
• Three causes of heat loss:
  • Radiant heat loss is the transfer of heat energy by infrared radiation from a warmer object to a cooler object
  • Conductive heat loss is the heat exchange by direct contact with colder material; it is usually a minor cause of heat loss, but it is more significant in persons exposed to a wet environment
  • Evaporative heat loss is the heat transfer from water to a gas by perspiration or respiration, which may be a significant factor for runners
• Hypothermia occurs faster during water-related activities, because water conducts heat to a greater degree than air.
• A person's ability to generate and maintain a normal physiological core temperature (35.5 to 37.2°C [98 to 98.6°F]) depends on adequate physical fitness, good nutritional status, adequate hydration, absence of injury, and proper clothing.

Evidence:

• Several studies and anecdotal experience indicate that outdoor enthusiasts and professional athletes benefit from education about hypothermia dangers and cold-water survival techniques (40; 41; 47).

Comments:

• Urban “weekend warriors” may not recognize the risk for hypothermia after participating in strenuous physical activity, such as a 5- or 10-km run. They are unlikely to appreciate the consequences of a cool breeze of 10°C (50°F) on perspiration-soaked clothes and to realize that this type of activity should be considered a “water-related” outdoor activity, thus increasing the risk for hypothermia.