Frozen Mythbusters: Myth #4 of 13.
There are a variety of myths regarding human response to cold exposure.  These myths are explained and debunked by Dr. Murray Hamlet, DMV, Dr. Gordon Giesbrecht, PHD, and Frank Hubbell, DO.  After posting the thirteen myths, a complete article  from the Wilderness Medicine Newsletter will be loaded for anyone interested in all the chilly little details.

Myth #4
              Rubbing frostbite is the proper, and best way, to rewarm frozen body parts. 
              For years this is one of several rewarming methods that has proven to not only be wrong, but also to be very detrimental to the patient. This myth goes along with several others that all deserve to be BUSTED at once. These include rubbing the frozen part with snow or ice and rewarming the frostbitten part with dry heat from a fire, stove top, hot exhaust air from an engine, or any drying device.
              Let’s take a finger, for example. When that finger begins to cool off, the local vasculature will vasoconstrict decreasing the circulation to that area. This reduction in circulating blood not only allows the area to cool faster but also prevents sufficient oxygen from getting to this tissue. The cold sensory nerves shut down causing a feeling of numbness followed by the loss of sensation. This initial phase is referred to as frostnip or 1st degree frostbite, and if caught at this point should be very easy to rewarm with the resultant return of full function.
               If this numbness or loss of sensation is ignored, the tissue will continue to cool off and eventually begin to freeze. As the water in the cells cools, it begins to expand. Water is the only known substance that expands as it cools to a solid and then continues to expand. Water is most dense at 4C (39F). As it continues to cool, it will continue to expand and freeze. (This is why ice floats.) As the water expands and cools, small ice crystals will form. If nothing is done about it at this point, the finger will go on to freeze solid. As the ice forms in the cells, it expands and bursts the cells. If the finger is rewarmed before it is solid, a large blister called a bleb will form with rewarming.
               The problem with rubbing cold, numb parts is the ice crystals.  If they have formed, as you massage the area, they will act like microscopic razor blades and shred the cells. Rubbing the cold tissue with snow only adds insult to injury and using hot air to rewarm will dehydrate the tissues and cause mummification. All equally bad for this cold finger.
               All of these rewarming techniques are busted. So, what should you do? First examine the area and determine if this is superficial or deep frostbite. Superficial, 1st or 2nd degree frostbite, will initially look the same. While the area will be numb and the skin pale or waxy due to vasoconstriction, the tissues will still be pliable. The difference between 1st and 2nd degree will become apparent once the areas have thawed out. 1st will return to normal, and 2nd will either appear bruised or more likely will form a large blister called a bleb.
              In the field superficial frostbite should be rewarmed with gentle skin-to-skin contact, where the cold area is placed against warm skin. Once rewarmed, it should be examined closely for bleb formation. If a bleb occurs, protect the area well as the bleb cannot be allowed to refreeze, since this will cause complete tissue destruction.
              Deep or 3rd degree frostbite is grossly obvious in that the tissue is ivory white and frozen solid. The area should be insulated to protect from further injury. We do not recommend that you try to field rewarm deep frostbite. The tissue damage has already occurred, and you can only make things worse.

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