Radiation

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  Radiation involves the direct emission or absorption of heat energy from the body (mostly infrared radiation).

  
  
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  Radiation is the largest source of heat loss from the body.

  
  
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  Clothed, sedentary individuals in a calm, temperate climate lose more body heat (approximately 60%) by radiation than active individuals (approximately 45%) in a thermoneutral environment where heat production equals heat loss.

  
  
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  The human body constantly radiates heat to nearby solid objects with a cooler temperature, and the rate of heat loss increases with the difference in temperature between the body and the object.

  
  
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  Radiant heat loss increases as temperature decreases but only becomes a significant concern in extremely cold environments.

  
  
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  Clothing does not markedly affect radiant heat loss.

Evaporation

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  Evaporation occurs when water or moisture is transformed into vapor.

  
  
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  Evaporation occurs with perspiration on the body’s surface and in respiratory passages as inspired air is warmed and moistened.

  
  
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  High altitude increases heat and water losses from the lungs because breathing deepens and the respiratory rate increases.

  
  
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  A small amount of “insensible” perspiration occurs at cold temperatures but may become significant in active individuals who are not appropriately dressed for that temperature.

  
  
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  Vigorous exercise with increased sweating leads to greater evaporative heat loss than with normal evaporative loss in sedentary individuals in a temperate climate (a difference of approximately 20%–30%).

  
  
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  Vapor barrier systems meant to decrease evaporative water loss are primarily ineffective in physically active individuals because they trap perspiration between the barrier and the skin and lead to increased heat loss by convection when the sweat rate exceeds the rate of permeability ( Fig. 22.1 ).

  
  

  
  

  
  

  

  
  

  
  

  Vapor barrier systems.

  
  
  
  
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  Wet clothing combined with wind leads to significant evaporative heat losses.

Convection

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  Convection involves the transfer of heat from the body to cold air or water in contact with the body surface.

  
  
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  Ongoing heat loss occurs when warmed air or water is continually displaced from the body surface and is replaced by air and water of colder temperatures.

  
  
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  The amount of heat loss that occurs by convection is determined by the temperature difference between the air and the body surface and by the speed of the air moving over the body.

  
  
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  Convective heat loss increases greatly with wind moving over the body surface, particularly with submersion in cold water.

  
  
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  In addition to natural wind, wind moving over the body surface created by cycling, skiing, running, windsurfing, or other activities that increase air flow over the body can cause marked heat loss, particularly at cold temperatures sustained over prolonged periods.

  
  
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  “Wind chill” is wind combined with cold that causes a lower “equivalent” temperature ( Fig. 22.2 ) than current environmental temperature and greatly accelerates heat loss by convection; wind chill is most significant with the first 20 mph increase in wind speed, with some additional effects at higher wind speeds.

  
  

  
  

  
  

  

  
  

  
  

  Wind chill chart.

  
  

  (From National Oceanic and Atmospheric Administration, National Weather Service [ http://www.nws.noaa.gov/om/winter/windchill.shtml ].)

  
  
  
  
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  Cold-water swimmers lose significant heat by convection because continuous motion prompts the ongoing displacement of water away from the body, resulting in constant exposure of a “warm” body to cold water.

  
  
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  Windproof clothing, particularly combined with appropriate insulation layers, can greatly reduce convective heat loss in most environments by trapping warm air and minimizing air displacement.

Conduction

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  Conduction involves the direct transfer of heat from the body when it is in contact with a surface colder than the body’s temperature (e.g., water, snow, ice, or rocks).

  
  
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  Water is an excellent conductor, and it has an exponentially greater volumetric heat capacity than air, which can lead to significant heat losses for anyone immersed in cold water, particularly without protective clothing.

  
  
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  In addition, wet clothing increases conductive heat losses.

Increase Heat Production

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  Increasing heat production is the least effective way to prevent cold illness.

  
  
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  Exercise: voluntary muscular activity will produce heat

  
  
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  Shivering: involuntary muscular activity will also produce heat

  
  
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  Eating: frequent meals or snacks are needed to replenish glycogen and fat stores, particularly with prolonged exercise in cold environments

  
  
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  Hydration: consume adequate fluids to prevent dehydration and subsequent impairment in circulating blood volume

Decrease Heat Loss

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  Decreasing heat loss is the most effective way to prevent cold illness.

  
  
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  Physiologic mechanisms for increasing heat production are much less effective than strategies applied to decrease heat loss.

  
  
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  Heat loss is primarily prevented with adequate layering of clothing.

  
  
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  Insulation and permeability are important properties of cold weather clothing.

  
  
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  Most versatile cold weather clothing systems are usually composed of three layers:

  
  
  
  
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    Inner hydrophobic polyester fabric (e.g., Capilene, Coolmax, Thermax, Thermolite, or Thermostat) that allows wicking of moisture away from the body. Avoid cotton.

    
    
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    Middle insulating material can be a second light layer (similar to inner layer fabric) or a heavier layer (can cause overheating; best used during exercise warm-up, cool-down, or in extremely cold environments) such as wool and wool/synthetic blends (heavy when wet but excellent insulator even then), pile and fleece (varying weights offer versatility for exercising athletes, fleece replacing pile), synthetic fillers (e.g., Primaloft, Liteloft, Dacron, Hollofill, Quallofil, or Thinsulate), or down (good in cold dry conditions, loses insulating properties when wet); the middle layer can lead to overheating.

    
    
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    Outer protective shell that is windproof and water repellant (e.g., newer treated nylons):

    
    
    
    
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      Newer treated nylons are best because they offer highest breathability: fibers are tightly woven, and sprays are used to increase water repellency.

      
      
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      Laminates such as Gore-Tex and other brand-specific materials designed to mimic Gore-Tex are advertised as “waterproof” and breathable, but the amount of sweat produced during exercise can exceed the capability of the laminate to transmit water vapor.

      
      
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      To date, an ideal fabric that allows water vapor to pass outward but not inward has not been developed.

      
      
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      Breathability of the outer layer is inversely proportional to the degree of water repellency and waterproofing; adequate breathability is important during exercise.

    
    

  
  
  
  
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  Middle and outer layers should each be slightly larger than the direct inner layer to allow a narrow space for warm air trapping; avoid “tight” layering.

  
  
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  The primary goal with layering is to stay relatively warm without excessively sweating or overheating, and layers must be shed or added depending on current activity levels and environmental conditions.

  
  
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  Ventilation ports in certain outer garments may be opened for adequate breathing and are particularly important when using laminates.

  
  
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  Excessive sweating causes heat loss through increased evaporation and conduction and may affect the insulating property of certain fabrics.

  
  
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  Wind chill can cause dangerously large amounts of heat loss through convection without windproof garments and appropriate insulation adjusted to activity levels.

Special Protection

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  Special protection and other measures to minimize heat loss and injury should be implemented.

  
  
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  Prevent significant conductive loss by placing a “barrier” between the body and colder objects (e.g., foam or other sleeping pads such as Therma-Rest, wet or dry suits with cold-water exposure, or leather or plastic boots with an inner insulating layer).

  
  
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  Maintain adequate trunk warmth to minimize vasoconstriction in hands and feet that may result in subsequent cold injury.

  
  
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  Wear mittens instead of gloves, preferably with a protective and windproof outer shell and an inner insulating layer.

  
  
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  Prevent radiant heat loss from the head (blood supply to the head is maintained in cold conditions) by wearing an insulating cap or balaclava and by using a hood when needed (high wind or wet weather).

  
  
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  Protect the exposed skin of face and ears by using a balaclava or neck gaiter pulled up over the face (neck gaiters also decrease heat loss from relatively superficial, large vessels of the neck).

  
  
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  Prevent genital injury by wearing undershorts with a windproof front panel.

  
  
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  Protect eyes (corneal freezing) by using ski goggles.

  
  
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  Avoid excessive wetting at all times, and if immersed in cold water, minimize movement and pull body into a “tight” position (assume the HELP posture— h eat e scape l essening p osture—which is similar to an “upright fetal” position) to decrease exposed surface area unless close to shore or a boat.

  
  
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  Adequately warm-up before exercising in cold conditions (may warm-up indoors or use insulated protective clothing if outdoors).

  
  
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  Avoid the use of emollients on the face because these have been shown to increase the incidence of frostbite and other cold injuries.

External Warming Sources

See Treatment discussion in the Accidental Hypothermia section.