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Hypothermia & Hyperthermia In Managing Motorcycle Riding Risk

By Craig Olson, M.D.
Originally Published in the IAMC Newsletter, June 2010

In the April issue of this newsletter we discussed the relationship of fatigue in the management of motorcycle riding risk. In this issue we will briefly review two more risks common to our sport – hypothermia and hyperthermia. Our marvelously created bodies have the ability to thermo regulate, remarkably maintaining our core body temperatures at 98.6 °F plus or minus 1.4 °F under a wide range of conditions. When our ability to thermo regulate is overwhelmed by thermal extremes and/or contributory factors, we can develop either hypothermia or hyperthermia, both of which can be potentially lethal.

Hypothermia is defined when the core body temperature drops below that required for normal metabolism (defined as 95.0 °F). This occurs when body heat is lost (usually due to exposure to cold) faster than it can be replenished. We lose body heat primarily through skin (90%) and lungs (10%) via four mechanisms: [1]

1. Radiation – due to thermal gradients between our body and any ambient environmental temperature below 98.6 °F. Largely the surface area of exposed body and the temperature gradient determines this. The more surface area exposed and the greater the gradient, the larger will be the loses in body heat due to radiation.

2. Conduction – due to molecular transference of heat energy through direct contact between objects. Water conducts heat away from the body 25 times faster than air because it has a greater density and therefore a great heat sink capacity. Generally conductive heat loss accounts for about 2% of overall loss. However, with wet clothes the loss is increased 5x.

3. Convection – a conductive process where one of the objects is in motion with its surface molecules being heated, moving away and replaced by new molecules that are also heated. The rate of convective heat loss depends on the density of the moving substance (water convection occurs faster than air convection) and the velocity of the moving substance. Wind chill is an example of the effects of air convection. A wind chill table gives a reading of the amount of heat lost to the environment relative to a still air temperature.

4. Evaporation – heat loss from converting water from a liquid to a gas. Perspiration, sweating and respiration are forms of evaporative heat loss. As body moisture is lost through these various evaporative processes, the overall circulating volume is reduced leading to dehydration, which makes the body more susceptible to hypothermia and other cold injuries.

Signs and symptoms depend on the degree of hypothermia. [1-3] Look for the “umbles” – stumbles, mumbles, fumbles, and grumbles which show changes in motor coordination and level of consciousness. In mild hypothermia (core temperature 98.6-96 °F) shivering is involuntary, ability to do complex motor functions is impaired, and the periphery is vasoconstricted (blanched skin). With moderate hypothermia (core temperature 96-93 °F), addition to the above, the victim will manifest a dazed appearance, loss of fine motor coordination, slurred speech, violent shivering, irrational behavior, and apathy to their condition.

With severe hypothermia (core temperature 9286 °F) shivering will occur in waves until it finally stops, they collapse – usually into a fetal position, pupils dilate, pulse rate decreases, and breathing becomes shallow. At a core body temperatures of 86 °F and below, the victim begins losing consciousness, breathing becomes erratic, and fatal cardiac arrhythmias develop.

Individuals developing hypothermia are also vulnerable to other cold-related injuries including: 1) frostbite (freezing of body tissues), 2) gangrene (death of tissues from interruption in blood flow), 3) chilblains (nerve and small blood vessel damage – usually in the hands or feet – following prolonged exposure to above freezing cold temperatures), 4) trench foot (damage to nerves and small blood vessels from prolonged immersion in cold water). [3]

Immediately seek medical attention for anyone who appears to have hypothermia. First-aid measures that can be initiated in the interim include: [1,3]

1. Reducing heat loss – add additional layers of dry clothing, remove any wet clothing, increase physical activity, and seek warmer shelter.

2. Adding fuel and fluids – carbohydrates provide the quickest source of energy for sudden brief heat surge. Maintain adequate hydration. Hot liquids can provide both calories and a heat source, but avoid caffeine, which is a diuretic and cardiac stimulant. Also avoid alcohol, which is a vasodilator causing increased peripheral heat loss, and tobacco/nicotine, which is a vasoconstrictor increasing the risk of frostbite.

3. Adding heat – fire or other external heat source. In cases of severe hypothermia handle the victim gently since any excessive, vigorous or jarring movements may trigger cardiac arrest. [2]

Hyperthermia is an elevated body temperature (above 100-101 °F) due to failed thermoregulation and occurs when the body produces or absorbs more heat than it can dissipate through the mechanisms enumerated above. When elevated body temperatures are sufficiently high, hyperthermia becomes a medical emergency requiring immediate treatment to prevent disability and death. [5]

A number of heat-related illnesses may develop in the progression of hyperthermia. [6] These include:

1. Sunburn – an injury to skin produced by overexposure to ultraviolet radiation from the sun’s rays and may result in first degree burns (reversible injury to epidermis – outer layer of skin – manifested as redness with mild swelling and minimal discomfort) or second degree burns (reversible injury to epidermis and dermis manifested by blister formation, intense splotchy redness and severe swelling and pain). Sunburned skin loses its ability to effectively dissipate heat thus increasing the risk of hyperthermia.

2. Heat rash – caused by excessive sweating during hot, humid weather and is manifested by a red cluster of pimples or small blisters usually occurring on the neck and upper chest, in the groin, under the breasts, and in elbow creases.

3. Heat cramps – muscle pains or spasms (usually in the abdomen, arms, or legs) that may occur with significant sweating during strenuous activity as a result of depletion of body fluids and salts.

4. Heat exhaustion – results as the body’s response to an excessive loss of water and salt in sweat and is manifest as heavy sweating, paleness, muscles cramps, tiredness, weakness, dizziness, headache, nausea or vomiting, and fainting. The victim’s skin may be cool and moist, the pulse rate may be rapid and weak, and breathing may be fast and shallow. Untreated, heat exhaustion may progress to heat stroke.

5. Heat stroke – a continuum of the above with the addition of confusion, seizures, and unconsciousness. Body temperature may rise to 106 °F or higher within 10-15 minutes. The skin is red, hot and moist or dry (no sweating), and the pulse is rapid and strong. Untreated, heat stroke can cause death or permanent disability

Treatment consists of moving the victim out of the sun and into a cooler shady place. Remove any excessive clothing and cool the body by covering it with damp sheets or by spraying it with cool water. Air may be directed onto the body with a fan or newspaper. If conscious, have the victim drink cool water or another nonalcoholic beverage without caffeine. [5-7] .

Prevention is the best medicine when it comes to dealing with hypothermia or hyperthermia. [8] Drinking plenty of fluids (non alcoholic and non caffeine) is fundamental to both conditions. The adequacy of hydration is best accessed by the amount and quality of your urine – it should be copious and clear rather than infrequent and concentrated (dark yellow). Replenish electrolytes depleted through sweating with fruit juices, sports beverages, or electrolyte substitutes (Elete Electrolyte Add-In©). On longer, multiday rides consider a hydration system (CamelBak or equivalent) for more consistent and constant rehydration. When in remote areas, consider carrying a water filtering/purification system to insure that you are not caught without drinkable water.

Protection from sunburn is important regardless of the ambient temperature. Use a sunscreen with SPF (sun protection factor) of 30 or greater, preferably one that blocks both UVA and UVB rays. [9]

Proper clothing and appropriate riding gear are the best protection from both cold and heat. Polypropylene is better at wicking moisture from perspiration and sweating away from the body facilitating cooling during hotter weather and preventing dampness during colder weather that may lead to excessive body cooling. Layers of cotton or wool outer clothing retain body heat better in the cold weather but impede heat dissipation during hotter temperatures. A baklava or face and neck mask along with windproof riding jacket and pants help prevent excessive heat loss. Extra warm gloves or heavier gloves with glove liners also help. An electric heating vest, gloves, seat and/or grips may be beneficial as well. Appropriate rain gear will keep you dry, preventing excessive heat loss from riding in wet clothes. In hot weather evaporative cooling vests and neckerchiefs along with mesh and vented riding jackets and pants help with dissipation of body heat.

Any riding, especially adventure riding, can be work. The more difficult the riding terrain, the more energy expended (and thus more body heat generated) riding it. Pace yourself, particularly in hot weather, and make frequent stops to rest (in a shady area if possible) thus allowing your body to adjust and dissipate excessive build up of body heat. It is best to use a buddy system when riding in either extreme of heat or cold.

Be prepared and plan for the unexpected. Complacency in riding preparation can get you into trouble. During a recent ride to Jordan Craters, I watched the ambient temperature drop over 30 ° (from 86 °F to 52 °F) in about 20 minutes as an unexpected thunderstorm came through. With only a riding jersey and no rain gear or extra layers of clothing, I got soaked and cold 30 miles from nowhere. Fortunately, the rain lasted only 20 minutes and by another 20 minutes the sun came back out, and the temperature rose to 78 °F. I was lucky and did not develop hypothermia, but I could have had the inclement weather conditions persisted longer. You will have more enjoyable and safer adventure riding by keeping aware of these two riding risks and preventively preparing for them.


1. Outdoor Action Guide to Hypothermia And Cold Weather Injuries, by Rick Curtis, ( ld.shtml)

2. Hypothermia – Mayo Clinic, (

3. Mayo Clinic Staff on Hypothermia, ( mia/ds00333)

4. Hypothermia, (

5. Hyperthermia, (

6. Hyperthermia and Heat-Related Illness, ( rticle.htm)

7. Treatment of Heat Stroke, Mayo Clinic, (

8. Hyperthermia, Health Encyclopedia, ( ain.html)

9. Sunburn, e-medicine health, ( ticle_em.htm)

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