A common injury forcing AR teams to drop from a race is hypothermia (even in summer - in fact the most typical scenario involves rain and temperatures in the 50s). Since this is a potentially life threatening condition, its important to recognize its causes and prevention, as well as the symptoms and treatment.
Causes. There are five recognized vectors for body heat transfer, which contribute in varying degrees to a loss in body temperature.
Factoid: did you know that the thermal conductivity of water is 240 times greater than that of air? Hal Weiss: "Secrets of Warmth"
The process leading towards hypothermia begins with exposure and exhaustion. When the body is exposed to the heat sucking factors listed above, particularly conduction and convection, and the rate of heat loss is greater than the body can produce - you're on your way to hypothermia. The pace of racing can exacerbate the process when your intake of fuel is insufficient to maintain both the body's temperature and the physical effort.
Prevention is worth a ton of cure. First and foremost, continually feed the machine during endurance events. Limit radiative heat loss by wearing a hat and dressing in insulating layers - remember its the volume of the trapped air - not the weight of the material that provides insulation! Limit conductive heat loss by wearing gloves while handling those aluminum trekking poles or climbing hardware (aluminum is an extremely efficient conductor); and wear garments that maintain an insulating quality even when wet (cotton T-shirts are best suited to polishing cars). Limit convection by wearing a windproof shell and covering exposed skin on windy days. Limit evaporative heat loss by preventing excessive perspiration before it starts - select clothing that is easily vented and avoid overdressing for the anticipated level of activity. Limiting respirative heat loss becomes important in the more extreme cold environments (10 degrees and below) - do this by wearing a balaclava or scarf.
Recognition. The symptoms of hypothermia begin with uncontrollable shivering (the body's physiological response to generate heat). When shivering stops, but is accompanied by symptoms resembling intoxication (such as clumsiness, slurred speech, and impaired mental agility) - watch out - this is the body making its next physiological response, having restricted its dwindling heat generation efforts to core systems. Those always-prepared Boy Scouts have posted a detailed breakdown of the physiological symptoms and associated body temperatures for hypothermia.
Remedial Action. The further the situation deteriorates, the less the likelihood an individual will be able to recover without significant assistance. In the first few stages, simply refueling and donning additional layers of insulation may be sufficient to stop the onset of hypothermia. This is where teammates can really make a difference, by keeping an eye on each other (rather than on the clock) for the early symptoms and taking immediate steps to remedy the situation (which may be as simple as stopping long enough to put on a windbreaker). A sudden, unexpected submersion in cold water is a common catalyst for hypothermia. Heat loss from conduction in these situations is rapid, but a quick recovery is often possible when your garments retain a high degree of loft (insulating, air trapping capacity) even when wet. If getting soaked is a real possibility, the winners are poly-fleece, scratchy old wool, and neoprene - the losers are cotton and skin-hugging spandex.
When hypothermia has advanced beyond the point of the violent shivering, extreme measures are necessary and its time to throw in the towel. The top priority needs to be getting the individual into a warm environment. If available, provide warm fluids. If the situation does not permit this, another option is for a non-hypothermic individual to share a sleeping bag, bivy sack, or space blanket with the hypothermic teammate until their body temperature returns to normal or help arrives.
Bottom Line: When heat loss becomes a problem - immediately take measures to insulate yourself from the sources of heat loss (conductive and convective being the worst); and get calories into your system fast!
For more on the subject of hypothermia, Adventure Sports Online has posted a more comprehensive discussion.
Frostbite, a somewhat less common cold weather injury than hypothermia, is caused when the vessels carrying blood to the tissues actually freezes. In minor stages, this is reversible, but more severe cases actually result in tissue death. Feet are the most commonly affected extremity (particularly the big toe), followed closely by hands. A fair number of cases also involve the face and ears. Note that when ambient temperatures are at or below freezing, and wind drives the wind chill factor down to -20F or lower… frostbite can occur within 1 minute to exposed flesh. The McKinley Health Center Web Site has posted a frostbite fact sheet that elaborates on cause and prevention
Excerpts from notes by Buck Tilton on the Wilderness Medicine Institute web site
Respiratory Discomfort. Hard exercise on a cold day, the kind of workout that requires aggressive panting, might lead to a second winter worry: "frozen lung." Temps must be low, usually below zero Fahrenheit. No tissue actually freezes, but severe bronchial irritation results from sucking down very cold air faster that the human airway can warm it up. The irritation produces spasms in the muscles of the airway and a burning pain, with the possibility of coughing up blood. Increased mucus production frequently creates wheezing sounds when the sufferer breathes. A severe case might last one to two weeks. Treatment is rest, warm and humidified air to breathe, and plenty of water to drink. Prevention is less painful: wear a hooded parka and a face mask, or breathe through a fluffy scarf.
Immersion Foot. If the wet and cold are isolated in boots, another non-freezing problem, immersion foot, might develop. Also called "trench foot," prolonged exposure to the wet and cold leads to lack of circulation in the feet. Owners of these feet complain of numbness, tingling pain, and itching. The feet look white or mottled in disgusting shades of blue, gray, and burgundy. On rewarming the pain does much more than tingle, and swelling and redness occur. Severe cases crack the skin and bacterial invasion is imminent. Avoid aggressive rewarming. Skin-to-skin is best. All cases need to be kept warm and dry. Walking may become difficult. Painkillers often help. William Forgey, MD, "Father of Wilderness Medicine", suggests an ounce of hard liquor every waking hour to promote circulation to the damaged feet until a doctor can be found.
Snow Blindness. Failure to wear sunglasses that keep all the ultraviolet light out, may lead to a fifth problem, snowblindness (sunburned eyes). There is seldom any sensation on the surface of the eye until the damage of too-much-sun is done. It can occur in as little as one hour. Usually six to twelve hours pass before the eye feels painful and dry and gritty. It hurts very much to move or open the eye--so don't do it. Rinse the eye with cold water, and patch it closed. The problem should resolve in 24-to-48 hours with no permanent damage. If it doesn't, or if the pain is unbearable, see a physician.
Sunburn. The winter sun is closer to the earth than the summer sun, but, because it's lower in the sky, the radiation is less intense. This healthier aspect of winter exposure is counterbalanced by the fact that snow and ice are very efficient reflectors of sunlight, bouncing 80-to-85% back on the outdoor enthusiast. Reflected light helps create snowblindness and another problem, "sun poisoning", an allergic reaction to ultraviolet light, primarily UVA. Allergies are specific to certain susceptible people, and this one looks like poison ivy, or, sometimes, eczema. Intense itching usually starts 24 - 48 hours after exposure. Treatment for most allergies is limited to time, anti-itch measures (topical hydrocortizone and anti-histamines), and eliminating contact with the allergy-producing agent. Severe reactions often benefit from prescription drugs that a doctor must suggest. Prevention is offered by clothing and sunscreens that block UVA and UVB radiation. Of particular efficacy are sunscreens that contain titanium dioxide.