How We Lose Heat to the Environment
- Radiation - loss of heat to the environment due to the
temperature gradient (this occurs only as long as the ambient
temperature is below 98.6). Factors important in radiant heat
loss are the surface area and the temperature gradient.
- Conduction - through direct contact between objects, molecular
transference of heat energ
- Water conducts heat away from the body 25 times faster
than air because it has a greater density (therefore a greater
heat capacity). Stay dry = stay alive!
- Steel conducts heat away faster than water
Example: Generally conductive heat loss accounts for
only about 2% of overall loss. However, with wet clothes the
loss is increased 5x.
Convection - is a process of conduction where one of the
objects is in motion. Molecules against the surface are heated,
move away, and are replaced by new molecules which are also
heated. The rate of convective heat loss depends on the density
of the moving substance (water convection occurs more quickly
than air convection) and the velocity of the moving substance.
Evaporation - heat loss from converting water from a liquid
to a gas
- Wind Chill - is an example of the effects of air convection,
the wind chill table gives a reading of the amount of heat
lost to the environment relative to a still air temperature.
- Perspiration - evaporation of water to remove excess
- Sweating - body response to remove excess heat
- Respiration - air is heated as it enters the lungs
and is exhaled with an extremely high moisture content
- It is important to recognize the strong connection
between fluid levels, fluid loss, and heat loss. As body
moisture is lost through the various evaporative processes
the overall circulating volume is reduced which can lead
to dehydration. This decrease in fluid level makes the
body more susceptible to hypothermia and other cold injuries.
Response to Cold
Cold Challenge - (negative factors)
- Wet (rain, sweat, water)
- Wind (blowing, moving, e.g. biking) Total = Cold Challenge
Heat Retention - (positive factors)
- Size/shape (Eskimo vs. Masai)
- Insulation (layering/type)
- Fat (as insulation)
- Shell/core (shunt blood to core) shell acts as a thermal
barrier. Total = Heat Retention
Heat Production - (positive factors)
- Exercise, shivering Limited by:
- Fuel stores (glycogen)
- Fluid status (efficient exercise)
- Food intake (kindling, sticks, logs) Total = Heat Production
| Heat Retention
Surface to Volume ratio
Shell to Core shunting
Your Body Core Temperature
- Heat is both required and produced at the cellular
level. The environment acts as either a heating or a cooling
force on the body. The body must be able to generate heat,
retain heat, and discharge heat depending on the body activity
and ambient external temperature.
- Body temperature is a measure of the metabolism - the
general level of chemical activity within the body.
- The hypothalamus is the major center of the brain for
regulating body temperature. It is sensitive to blood temperature
changes of as little as 0.5 degrees Celsius and also reacts
to nerve impulses received from nerve endings in the skin.
- The optimum temperature for chemical reactions to take
place in the body is 98.6 degrees F. Above 105 F many body
enzymes become denatured and chemical reactions cannot take
place leading to death. Below 98.6 F chemical reactions slow
down with various complications which can lead to death.
- Core = the internal body organs, particularly the heart,
lungs, and brain.
Periphery = the appendages, skin, and muscle tissue.
- Core temperature is the temperature that is essential
to the overall metabolic rate of the body. The temperature
of the periphery is not critical.
How Your Body Regulates Core Temperature
- Vasodilation - increases surface blood flow, increases
heat loss (when ambient temperature is less that body temperature).
Maximal vasodilation can increase cutaneous blood flow to
3000 ml/minute (average flow is 300-500 ml/minute).
- Vasoconstriction - decreases blood flow to periphery,
decreases heat loss. Maximal vasoconstriction can decrease
cutaneous blood flow to 30 ml/minute.
- Sweating - cools body through evaporative cooling
- Shivering - generates heat through increase in chemical
reactions required for muscle activity. Visible shivering
can maximally increase surface heat production by 500%. However,
this is limited to a few hours because of depletion of muscle
glucose and the onset of fatigue.
- Increasing/Decreasing Activity will cause corresponding
increases in heat production and decreases in heat production.
- Behavioral Responses - putting on or taking off layers
of clothing will result in heat regulation
1. Hypothermia - "a decrease in the core body temperature
to a level at which normal muscular and cerebral functions
are impaired." - Medicine for Mountaineering
2. Conditions Leading to Hypothermia
- Cold temperatures
- Improper clothing and equipment
- Fatigue, exhaustion
- Poor food intake
- No knowledge of hypothermia
- Alcohol intake - causes vasodilation leading to increased
3. What are "hypothermia" temperatures
- Below freezing
- 40 degrees - Ex. Shenandoahs, wind and rain
- 60 degrees - Ex. Rayanna and hurricane
- Any temperature less than 98.6 degrees can be linked
to hypothermia (ex. hypothermia in the elderly in cold houses)
or peripheral circulation problems such as trench foot and
4. Signs and Symptoms of Hypothermia
a. Watch for the "-Umbles" - stumbles, mumbles, fumbles,
and grumbles which show changes in motor coordination and
levels of consciousness
b. Mild Hypothermia - core temperature 98.6 - 96 degrees
- Shivering - not under voluntary control
- Can't do complex motor functions (ice climbing or
skiing) can still walk & talk
- Vasoconstriction to periphery
c. Moderate Hypothermia - core temperature 95 - 93 degrees
- Dazed consciousness
- Loss of fine motor coordination - particularly in
hands - can't zip up parka, due to restricted peripheral
- Slurred speech
- Violent shivering
- Irrational behavior - Paradoxical Undressing - person
starts to take off clothing, unaware s/he is cold
- "I don't care attitude" - flattened affect
d. Severe Hypothermia - core temperature 92 - 86 degrees
and below (immediately life threatening)
- Shivering occurs in waves, violent then pause, pauses
get longer until shivering finally ceases - because the
heat output from burning glycogen in the muscles is not
sufficient to counteract the continually dropping core
temperature, the body shuts down on shivering to conserve
- Person falls to the ground, can't walk, curls up
into a fetal position to conserve heat
- Muscle rigidity develops - because peripheral blood
flow is reduced and due to lactic acid and CO2 buildup
in the muscles
- Skin is pale
- Pupils dilate
- Pulse rate decreases
- at 90 degrees the body tries to move into hibernation,
shutting down all peripheral blood flow and reducing breathing
rate and heart rate.
- at 86 degrees the body is in a state of "metabolic
icebox." The person looks dead but is still alive.
e. Death from Hypothermia
- Breathing becomes erratic and very shallow
- Cardiac arrythmias develop, any sudden shock may
set off Ventricular Fibrillation
- Heart stops, death
5. How to Assess if someone is Hypothermic
If shivering can be stopped voluntarily = mild hypothermia
- Ask the person a question that requires higher reasoning
in the brain (count backwards from 100 by 9's). If the person
is hypothermic, they won't be able to do it. [Note: there
are also other conditions such as altitude sickness that
can also cause the same condition.]
- If shivering cannot be stopped voluntarily = moderate
- severe hypothermia
- If you can't get a radial pulse at the wrist it indicates
a core temp below 90 - 86 degrees
- The person may be curled up in a fetal position. Try
to open their arm up from the fetal position, if it curls
back up, the person is alive. Dead muscles won't contract
only live muscles.
Signs & Symptoms
99º - 97ºF
Normal, shivering can begin
97º - 95ºF
Cold sensation, goose bumps, unable to perform
complex tasks with hands, shiver can be mild to severe,
95º - 93ºF
Shivering, intense, muscle incoordination becomes
apparent, movements slow and labored, stumbling pace,
mild confusion, may appear alert. Use sobriety test,
if unable to walk a 30 foot straight line, the person
93º - 90ºF
Violent shivering persists, difficulty speaking,
sluggish thinking, amnesia starts to appear, gross
muscle movements sluggish, unable to use hands, stumbles
frequently, difficulty speaking, signs of depression,
90º - 86ºF
Shivering stops, exposed skin blue of puffy, muscle
coordination very poor, inability to walk, confusion,
incoherent/irrational behavior, but may be able to
maintain posture and appearance of awareness
86º - 82ºF
Muscle rigidity, semiconscious, stupor, loss of
awareness of others, pulse and respiration rate decrease,
possible heart fibrillation
82º - 78ºF
Unconscious, heart beat and respiration erratic,
pulse may not be palpable
78º - 75ºF
Pulmonary edema, cardiac and respiratory failure,death.
Death may occur before this temperature is reached.
The basic principles of rewarming a hypothermic victim
are to conserve the heat they have and replace the body fuel
they are burning up to generate that heat. If a person is
shivering, they have the ability to rewarm themselves at a
rate of 2 degrees C per hour.
Mild - Moderate Hypothermia
- Reduce Heat Loss
Add Fuel & Fluids
- Additional layers of clothing
- Dry clothing
- Increased physical activity
It is essential to keep a hypothermic person adequately
hydrated and fueled.
a. Food types
- Carbohydrates - 5 calories/gram - quickly released
into blood stream for sudden brief heat surge - these
are the best to use for quick energy intake especially
for mild cases of hypothermia
- Proteins - 5 calories/gram - slowly released - heat
given off over a longer period
- Fats - 9 calories/gram - slowly released but are
good because they release heat over a long period, however,
it takes more energy to break fats down into glucose -
also takes more water to break down fats leading to increased
b. Food intake
- Hot liquids - calories plus heat source
- Sugars (kindling)
- GORP - has both carbohydrates (sticks) and proteins/fats
c. Things to avoid
- Alcohol - a vasodilator - increases peripheral heat
- Caffeine - a diuretic - causes water loss increasing
- Tobacco/nicotine - a vasoconstrictor, increases risk
- Fire or other external heat source
- Body to body contact. Get into a sleeping back, in
dry clothing with a normothermic person in lightweight dry
- Reduce Heat Loss
Add Fuel & Fluids
- Hypothermia Wrap: The idea is to provide a shell of
total insulation for the patient. No matter how cold, patients
can still internally rewarm themselves much more efficiently
than any external rewarming. Make sure the patient is dry,
and has a polypropylene layer to minimize sweating on the
skin. The person must be protected from any moisture in
the environment. Use multiple sleeping bags, wool blankets,
wool clothing, Ensolite pads to create a minimum of 4" of
insulation al the way around the patient, especially between
the patient and the ground. Include an aluminum "space"
blanket to help prevent radiant heat loss, and wrap the
entire ensemble in plastic to protect from wind and water.
If someone is truly hypothermic, don't put him/her naked
in a sleeping bag with another person.
- Warm Sugar Water - for people in severe hypothermia,
the stomach has shut down and will not digest solid food
but can absorb water and sugars. Give a dilute mixture of
warm water with sugar every 15 minutes. Dilute Jello™
works best since it is part sugar and part protein. This
will be absorbed directly into the blood stream providing
the necessary calories to allow the person to rewarm themselves.
One box of Jello = 500 Kilocalories of heat energy. Do
not give full strength Jello even in liquid form, it
is too concentrated and will not be absorbed.
- Urination - people will have to urinate from cold diuresis.
Vasoconstriction creates greater volume pressure in the
blood stream. The kidneys pull off excess fluid to reduce
the pressure. A full bladder is a place for additional heat
loss so urinating will help conserve heat. You will need
to help the person urinate. Open up the Hypothermia Wrap
enough to do this and then cover them back up. You will
need to keep them hydrated with the dilute Jello solution
Heat can be applied to transfer heat to major arteries
- at the neck for the carotid, at the armpits for the brachial,
at the groin for the femoral, at the palms of the hands
for the arterial arch.
- Chemical heat packs such as the Heat Wave™
provides 110 degrees F for 6-10 hours.
- Hot water bottles, warm rocks, towels, compresses
- For a severely hypothermic person, rescue breathing
can increase oxygen and provide internal heat.
Is a situation in which the core temperature actually
decreases during rewarming. This is caused by peripheral
vessels in the arms and legs dilating if they are rewarmed.
This dilation sends this very cold, stagnate blood from
the periphery to the core further decreasing core temperature
which can lead to death. In addition, this blood also is
very acetic which may lead to cardiac arrythmias and death.
Afterdrop can best be avoided by not rewarming the periphery.
Rewarm the core only! Do not expose a severely hypothermic
victim to extremes of heat.
CPR & Hypothermia
When a person is in severe hypothermia they may demonstrate
all the accepted clinical signs of death:
- Blue skin
- Fixed and dilated pupils
- No discernable pulse
- No discernable breathing
- Comatose & unresponsive to any stimuli
- Rigid muscles
But they still may be alive in a "metabolic icebox"
and can be revived. You job as a rescuer is to rewarm the
person and do CPR if indicated. A hypothermia victim is
never cold and dead only warm and dead. During severe hypothermia
the heart is hyperexcitable and mechanical stimulation (such
as CPR, moving them or Afterdrop) may result in fibrillation
leading to death. As a result CPR may be contraindicated
for some hypothermia situations:
- Make sure you do a complete assessment of heart
rate before beginning CPR. Remember, the heart rate
may be 2-3/minute and the breathing rate 1/30 seconds. Instituting
cardiac compressions at this point may lead to life-threatening
arrythmias. Check the carotid pulse for a longer time period
(up to a minute) to ascertain if there is some slow heartbeat.
Also, even though the heart is beating very slowly, it is
filling completely and distributing blood fairly effectively.
External cardiac compressions only are 20-30% effective.
Thus, with its severely decreased demands, the body may
be able to satisfy its circulatory needs with only 2-3 beats
per minute. Be sure the pulse is absent before beginning
CPR. You will need to continue to do CPR as you rewarm the
- Ventilation may have stopped but respiration may
continue - the oxygen demands for the body have been so
diminished with hypothermia that the body may be able to
survive for some time using only the oxygen that is already
in the body. If ventilation has stopped, artificial ventilation
may be started to increase available oxygen. In addition,
blowing warm air into the persons lungs may assist in internal
- CPR Procedures
- Check radial pulse, between 91.4 and 86 degrees F
this pulse disappears
- Check for carotid pulse - wait at least a full minute
to check for very slow heartbeat
- If pulse but not breathing or slow breathing, give
rescue breathing (also adds heat).
- If no discernible heartbeat begin CPR and be prepared
to continue - persons with hypothermia have been given
CPR for up to 3.5 hours and have recovered with no
- Begin active rewarming
Tissue temperature in cold weather is regulated by two
factors, the external temperature and the internal heat
flow. All cold injuries described below are intimately connected
with the degree of peripheral circulation. As peripheral
circulation is reduced to prevent heat loss to the core
these conditions are more likely to occur.
- Factors influencing cold injuries
Cold-induced Vasodilation - When a hand or foot is
cooled to 59 degrees F, maximal vasoconstriction and minimal
blood flow occur. If cooling continues to 50 degrees, vasoconstriction
is interrupted by periods of vasodilation with an increase
in blood and heat flow. This "hunting" response recurs in
5-10 minute cycles to provide some protection from cold.
Prolonged, repeated exposure increases this response and
offers some degree of acclimatization. Ex. Eskimos have
a strong response with short intervals in between.
Pathophysiology of Tissue Freezing - As tissue begins
to freeze, ice crystals are formed within the cells. As
intracellular fluids freeze, extracellular fluid enters
the cell and there is an increase in the levels of extracellular
salts due to the water transfer. Cells may rupture due to
the increased water and/or from tearing by the ice crystals.
Do not rub tissue; it causes cell tearing from the ice
crystals. As the ice melts there is an influx of salts
into the tissue further damaging the cell membranes. Cell
destruction results in tissue death and loss of tissue.
Tissue can't freeze if the temperature is above 32 degrees
F. It has to be below 28 degrees F because of the salt content
in body fluids. Distal areas of the body and areas with
a high surface to volume ratio are the most susceptible
(e.g ears, nose, fingers and toes - this little rhyme should
help remind you what to watch out for in yourself and others).
- Low ambient temperature
- Wind chill - increases rate of freezing dramatically
- Moisture - wet skin freezes at a higher temp than
- Contact with metal or supercooled liquids (white
- Exposed skin
- Previous cold injuries
- Constricting garments
- Local pressure
- Cramped position
- Body type
- Women do better in cold than men (greater subcutaneous
- Caloric intake
- Diabetes, some medications
- Caffeine, nicotine
- Surface frostbite generally involves destruction
of skin layers resulting in blistering and minor tissue
loss. Blisters are formed from the cellular fluid released
when cells rupture.
- Deep frostbite can involve muscle and bone
May have sensation
- Circulation is reduce to the are to prevent heat
- The area may be pale, cold.
- It may have sensation or be numb.
- Freezing of top layers of skin tissue
- It is generally reversible
- White, waxy skin, top layer feels hard, rubbery but
deeper tissue is still soft
- Most typically seen on cheeks, earlobes, fingers,
- Rewarm the area gently, generally by blowing warm
air on it or placing the area against a warm body part
(partner's stomach or armpit)
- Do not rub the area - this can damage the
effected tissue by having ice crystals tear the cell
- Skin is white and "wooden" feel all the way through
- Superficial frostbite includes all layers of skin
- Numbness, possible anesthesia
- Deep frostbite can include freezing of muscle and/or
bone, it is very difficult to rewarm the appendage without
some damage occurring
Rewarming of Frostbite
- Superficial frostbite may be rewarmed as frostnip
if only a small area is involved
- If deep frostbite, see below for rewarming technique
Special Considerations for Frostbite
- Rewarming is accomplished by immersion of the effected
part into a water bath of 105 - 110 degrees F. No hotter
or additional damage will result. This is the temperature
which is warm to your skin. Monitor the temperature carefully
with a thermometer. Remove constricting clothing. Place
the appendage in the water and continue to monitor the
water temperature. This temperature will drop so that
additional warm water will need to be added to maintain
the 105 - 110 degrees. < >Do not add this warm water directly
to the injury. The water will need to be circulated fairly
constantly to maintain even temperature. The effected
appendage should be immersed for 25 - 40 minutes. Thawing
is complete when the part is pliable and color and sensation
has returned. Once the area is rewarmed, there can be
significant pain. Discontinue the warm water bath when
thawing is complete.
- Do not use dry heat to rewarm. It cannot be effectively
maintained at 105 - 110 degrees and can cause burns further
damaging the tissues.
- Once rewarming is complete the injured area should
be wrapped in sterile gauze and protected from movement
and further cold.
- Once a body part has been rewarmed it cannot be
used for anything. Also it is essential that the part
can be kept from refreezing. Refreezing after rewarming
causes extensive tissue damage and may result in loss
of tissue. If you cannot guarantee that the tissue
will stay warm, do not rewarm it. Mountaineers
have walked out on frozen feet to have them rewarmed after
getting out with no tissue loss. Once the tissue is frozen
the major harm has been done. Keeping it frozen will not
cause significant additional damage.
Trench Foot - Immersion Foot
- If the person is hypothermic and frostbitten, the
first concern is core rewarming. Do not rewarm the frostbitten
areas until the core temp approaches 96 degrees.
- No alcohol - vasodilation may increase fluid buildup
- No smoking - nicotine as a vasoconstrictor may increase
chances for developing frostbite
- Liquids such as white gas can "supercool" in the
winter (drop below their freezing point but not freeze).
White gas also evaporates quickly into the air. Spilling
supercooled white gas on exposed skin leads to instant
frostbite from evaporative cooling. Always were gloves
when handling fuel.
- Touching metal with bare skin can cause the moisture
on your skin to freeze to the metal. (In really cold conditions,
metal glasses frames can be a problem). When you pull
away, you may leave a layer of skin behind. Don't touch
metal with bare skin.
Trench foot is a process similar to chillblains. It
is caused by prolonged exposure of the feet to cool, wet
conditions. This can occur at temperatures as high as 60
degrees F if the feet are constantly wet. This can happen
with wet feet in winter conditions or wet feet in much warmed
conditions (ex. sea kayaking). The mechanism of injury is
as follows: wet feet lose heat 25x faster than dry, therefore
the body uses vasoconstriction to shut down peripheral circulation
in the foot to prevent heat loss. Skin tissue begins to
die because of lack of oxygen and nutrients and due to buildup
of toxic products. The skin is initially reddened with numbness,
tingling pain, and itching then becomes pale and mottled
and finally dark purple, gray or blue. The effected tissue
generally dies and sloughs off. In severe cases trench foot
can involve the toes, heels, or the entire foot. If circulation
is impaired for > 6 hours there will be permanent damage
to tissue. If circulation is impaired or > 24 hours the
victim may lose the entire foot. Trench Foot causes permanent
damage to the circulatory system making the person more
prone to cold related injuries in that area. A similar phenomenon
can occur when hands are kept wet for long periods of time
such as kayaking with wet gloves or pogies. The damage to
the circulatory system is known as Reynaud's Phenomenon.
Treatment and Prevention of Trench foot
- Includes careful washing and drying of the feet,
gentle rewarming and slight elevation. Since the tissue
is not frozen as in severe frostbite it is more
susceptible to damage by walking on it. Cases of trench
foot should not walk out; they should be evacuated by
litter. Pain and itching are common complaints. Give Ibuprofen
or other pain medication.
- Prevention is the best approach to dealing with trench
foot. Keep feet dry by wearing appropriate footwear. Check
your feet regularly to see if they are wet. If your feet
get wet (through sweating or immersion), stop and dry
your feet and put on dry socks. Periodic air drying, elevation,
and massage will also help. Change socks at least once
a day and do not sleep with wet socks. Be careful of tight
socks which can further impair peripheral circulation.
Foot powder with aluminum hydroxide an help. High altitude
mountaineers will put antiperspirant on their feet for
a week before the trip. The active ingredient, aluminum
hydroxide will keep your feet from sweating for up to
a month and their are no confirmed contraindications for
wearing antiperspirant. [Some studies have shown links
between aluminum in the body and Alzheimers.] Vapor barrier
socks may increase the possibility of trenchfoot. When
you are active and you are wearing a vapor barrier sock,
you must carefully monitor how you sweat. If you are someone
who sweats a lot with activity, your foot and polypropylene
liner sock may be totally soaked before the body shuts
down sweating. Having this liquid water next to the skin
is going to lead to increased heat loss. If you don't
sweat much, your body may shut down perspiration at the
foot before it gets actually wet. This is when the vapor
barrier system is working. You must experiment to determine
if vapor barrier systems will work for you.
Avoiding Frostbite and Cold related Injuries
- Caused by repeated exposure of bare skin to temperatures
below 60 degrees
- Redness and itching of the effected area
- Particularly found on cheeks and ears, fingers and
- Women and young children are the most susceptible
- The cold exposure causes damage to the peripheral
capillary beds, this damage is permanent and the redness
and itching will return with exposure
- "Buddy system" - keep a regular watch on each other's
faces, cheeks, ears for signs of frostnip/frostbite
- Keep a regular "self check" for cold areas, wet feet,
numbness or anesthesia
- If at any time you discover a cold injury, stop
and rewarm the area (unless doing so places you at greater
a. Freezing of Cornea
- Caused by forcing the eyes open during strong winds
- Treatment is very controlled, rapid rewarming e.g.
placing a warm hand or compress over the closed eye.
After rewarming the eyes must be completely covered
with patches for 24 - 48 hours.
b. Eyelashes freezing together
- Put hand over eye until ice melts, then can open
- Sunburn of the eyes
- Prevention by wearing good sunglasses with side shields
or goggles. Eye protection from sun is just as necessary
on cloudy or overcast days as it is in full sunlight when
you are on snow. Snow
blindness can even occur during a snow storm if the cloud
cover is thin.
- Occur 8-12 hours after exposure
- Eyes feel dry and irritated, then feel as if they
are full of sand, moving or blinking becomes extremely
painful, exposure to light hurts the eyes, eyelids may
swell, eye redness, and
- Cold compresses and dark environment
- Do not rub eyes
- Hypothermia: Causes, Effects, and Prevention,
Robert Pozos, David Born, New Century, 1982.
- Management of Wilderness and Environmental Emergencies,
Paul Auerbach, Edward Geehr, Macmillan, 1983.
- Medicine for Mountaineering, James Wilkerson,
The Mountaineers, 1992.
- Hypothermia - Death by Exposure, William Forgey,
- Hypothermia, Frostbite, and other Cold Injuries,
James Wilkerson, Cameron Bangs, John Hayward, The Mountaineers,
- Medicine for the Backcountry, Buck Tilton
and Frank Hubbell, ICS Books, 1994.
This article is written by Rick Curtis, Director,
Outdoor Action Program. This material may be freely distributed
for nonprofit educational use. However, if included in publications,
written or electronic, attributions must be made to the
author. Commercial use of this material is prohibited without
express written permission from the author. Copyright ©
1995 Rick Curtis, Outdoor Action Program, Princeton University.
Disclaimer and Reproduction Information: Information in
NASD does not represent NIOSH policy. Information included in
NASD appears by permission of the author and/or copyright holder.