Cooling of body parts may result in various cold injuries - nonfreezing injuries, freezing injuries - and hypothermia – during which the core body temperature falls to a dangerous level. Nonfreezing cold injuries include chilblain, immersion foot and trenchfoot. Frostnip and frostbite are freezing injuries.
Toes, fingers, ears and nose are at greatest risk because these are most exposed and do not have major muscles to produce heat. The body will preserve heat by favouring the internal organs and thus reducing the flow of blood to the extremities under cold conditions. Hands and feet tend to get cold more quickly than the torso because:
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they lose heat more rapidly since they have a higher surface area-to-volume ratio, and
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they are more likely to be in contact with colder surfaces than other parts of the body.
If the eyes are not protected with goggles in high wind chill conditions, the corneas of the eyes may freeze.
Immersion foot and trenchfoot are "wet cold disease" resulting from prolonged exposure in a damp or wet environment at low temperatures. Depending on the temperature, an onset of symptoms may range from several hours to many days but the average is three days. Trenchfoot is more likely to occur at lower temperatures whereas an immersion foot is more likely to occur at higher temperatures and longer exposure times. A similar condition of the hands can occur if a person wears wet gloves for a prolonged period under cold conditions described above.
Frostnip is the mildest form of a freezing cold injury. It occurs when ear lobes, noses, cheeks, fingers, or toes are exposed to the cold and the top layers of a skin freeze. The skin of the affected area turns white and it may feel numb. The top layer of skin feels hard but the deeper tissue still feels normal (soft).
Frostbite is a common injury caused by exposure to extreme cold or by contact with extremely cold objects (especially those made of metal). It may also occur in normal temperatures from contact with cooled or compressed gases. Frostbite occurs when tissue temperature falls below the freezing point (0°C/32°F), or when blood flow is obstructed. Blood vessels may be severely and permanently damaged, and blood circulation may stop in the affected tissue. In mild cases, the symptoms include inflammation of the skin in patches accompanied by slight pain. In severe cases, there could be tissue damage without pain, or there could be burning or prickling sensations resulting in blisters. Frostbitten skin is highly susceptible to infection, and gangrene (local death of soft tissues due to loss of blood supply) may develop.