Heat injuries can be immediately life-threatening. Be aware of the temperature conditions and your hydration levels. The information provided here is designed for educational use only and is not a substitute for specific training or experience. Princeton University and the author assume no liability for any individual's use of or reliance upon any material contained or referenced herein. This article is prepared to provide basic information about heat related illnesses for the lay person. Medical research is always expanding our knowledge of the causes and treatment. It is your responsibility to learn the latest information. The material contained in this article may not be the most current. Copyright © 1997 Rick Curtis, Outdoor Action Program, Princeton University.
All the body's fluids make up one large body fluid pool. Losses of fluid
from any one source is reflected in the levels of all the body's other fluids:
e.g. profuse sweating will ultimately result in decreased blood volume.
If a patient loses enough fluid through any manner-bleeding, sweating, vomiting,
or diarrhea-the end result is the same: dehydration and, potentially, volume
shock. Adequate fluid is also critically important in hot environments to
help our body thermoregulate (see Heat Illnesses page 00). Remember, dehydration
can kill!
If someone is chronically losing fluid (from diarrhea or vomiting), then
you have a real emergency on your hands. Treat the cause of the fluid lose
as best you can (see Shock page 00, Bleeding page 00, Heat Illnesses page
00, Abdominal Infections page 00) and rehydrate the patient. Be prepared
to evacuate your patient.
Dehydration is always easier to prevent than it is to treat. So it is important to ensure that all members of your group replace their regular fluid losses by drinking adequate amounts of water (see below). Your body absorbs fluids best when you drink frequently and in small amounts rather than drinking large amounts at one time. It also helps with fluid absorption if you drink while eating. A pinch of salt and sugar in the water will do if no food is available. Very dilute mixtures of sports drinks like Gatorade® (add just enough to taste) work well for this purpose.
Don't depend on feeling thirsty to tell you when to drink. Thirst is
a late response of the body to fluid depletion. Once you feel thirsty, you
are already low on fluids. The best indicator of proper fluid levels is
urine output and color. You, and all the people in your group should strive
to be "copious and clear." Ample urine that is light colored to
clear shows that the body has plenty of fluid. Dark urine means that the
body is low on water, and is trying to conserve its supply by hoarding fluid
which means that urine becomes more concentrated (thereby darker).
| Season/Weather | Quarts/day | Explanation |
| Fall & Spring Backpacking* | 2-3 quarts 1.8-2.8 liters |
This is what an average person will need on a daily basis in general temperate conditions. |
| Hot Weather Backpacking* | 3-4 quarts 2.8-3.7 liters |
In hot and humid weather you are losing additional fluid through sweating which must be replaced. |
| Winter Backpacking* | 3-4 quarts 2.8-3.7 liters |
In the winter time you are losing moisture through evaporation to the dry air and especially through respiration. Dry air entering the lungs heats up and is exhaled saturated with moisture. |
| *All Seasons | Add 1quart 1.8 liters |
At high altitude the body looses more fluid. Increase your fluid intake if you are traveling at high altitudes (over 8,000 feet/2,438 meters) |
Another factor in overall fluid balance is the replacement of salts lost
to sweat. In most cases the salts found in normal food consumption is adequate
for salt replacement. In the event of severe dehydration, a solution of
½ teaspoon salt and ½ teaspoon of baking soda per quart/liter
of water can be used to replace lost fluid and salt. Use lukewarm fluids.
Discontinue the fluids if the person becomes nauseated or vomits. Restart
fluids as soon as the person can tolerate it.
The body has a number of mechanisms to properly maintain its optimal
core temperature of 98.6° F (37° C). Above 105° F (40° C)
many body enzymes become denatured and chemical reactions cannot take place
leading to death. Below 98.6° F (37° C) chemical reactions slow
down with various complications which can lead to death. Understanding thermoregulation
is important to understanding Heat Illnesses and Cold Injuries.
Whenever you go into an environment that is less than your body temperature, you are exposed to a Cold Challenge. As long as your levels of Heat Production and Heat Retention are greater than the Cold Challenge, then you will be thermoregulating properly. If the Cold Challenge is greater than your combined Heat Production and Heat Retention, then you susceptible to a cold illness such as hypothermia or frostbite (see Table 9.3).
| Heat Retention | Heat Production | Cold Challenge | Cold Injury | |||
| Body Size/shape Insulation Body Fat Body shunting blood to the core |
Exercise Shivering |
Temperature Wetness Wind |
Hypothermia Frostbite |
Wind Chill can have a major impact on heat loss through convection (see
Chapter 2 - Equipment: Regulating Your Body Temperature). As air heated
by your body is replaced with cooler air pushed by the wind, the amount
of heat you can lose in a given period of time increases. This increase
is comparable to the amount of heat you would lose at a colder temperature
with no wind. The Wind Chill factor is a scale that shows the equivalent
temperature given a particular wind speed.
In hot weather, especially with and humidity, you can lose a great deal
of body fluid through exercise. This can lead to a variety of heat related
illnesses including Heat Exhaustion and Heat Stroke. Heat Challenge is a
combination of a number of external heat factors. Balanced against this
Heat Challenge is your body's methods of Heat Loss (passive and active).
When Heat Challenge is greater than Heat Loss, you are at risk for a heat-related
injury (see Table 9.4). In order to reduce the risk you need to either decrease
the Heat Challenge or increase your Heat Loss. Fluids are a central part
of exercising in a Heat Challenge (see Fluids above).
| Passive Heat Loss | Active Heat Loss | Heat Challenge | Heat Injury | |||
| Body Size/shape Insulation Body Fat Body shunting blood to the core |
Radiant Heat Sweating |
Temperature Exercise Humidity Body Wetness Wind |
Heat Syncope Heat Exhaustion Heat Stroke |
Ambient temperature is not the only factor that plays a role in creating the potential for heat injuries, humidity is also important. Since our bodies rely on the evaporation of sweat as a major method of cooling, high humidity reduces our ability to cool the body, increasing the risk of heat illnesses. The Heat Index shows the relative effects of temperature and humidity (see Table 9.5).
| 70º (21) | 75º(24) | 80º(27) | 85º(29) | 90º(32) | 95º(35) | 100º(38) | 105º(41) | 110º(43) | 115º(46) | 120º(49) | |
| Relative Humidity | |||||||||||
| 0% | 64º(18) | 69º(20) | 73º(23) | 78º(26) | 83º(28) | 87º(31) | 91º(33) | 95º(35) | 99º(37) | 103º(39) | 107º(42) |
| 10% | 65º(18) | 70º(21) | 75º(24) | 80º(27) | 85º(29) | 90º(33) | 95º(35) | 100º(38) | 105º(41) | 111º(44) | 116º(47) |
| 20% | 66º(19) | 72º(22) | 77º(25) | 82º(28) | 87º(30) | 93º(33) | 99º(37) | 105º(41) | 112º(44) | 120º(49) | 130º(54) |
| 30% | 67º(19) | 73º(23) | 78º(26) | 84º(29) | 90º(33) | 96º(36) | 104º(40) | 113º(45) | 123º(51) | 135º(57) | 148º(64) |
| 40% | 68º(20) | 74º(23) | 79º(26) | 86º(30) | 93º(34) | 101º(38) | 110º(43) | 123º(56) | 137º(58) | 151º(66) | |
| 50% | 69º(20) | 75º(24) | 81º(27) | 88º(31) | 96º(36) | 107º(42) | 120º(49) | 135º(57) | 150º(66) | ||
| 60% | 70º(21) | 76º(24) | 82º(28) | 90º(33) | 100º(38) | 114º(46) | 132º(56) | 149º(65) | |||
| 70% | 70º(21) | 77º(25) | 85º(29) | 93º(34) | 106º(41) | 124º(51) | 144º(62) | ||||
| 80% | 71º(22) | 78º(26) | 86º(30) | 97º(36) | 113º(45) | 136º(58) | |||||
| 90% | 71º(22) | 79º(26) | 88º(31) | 102º(39) | 122º(50) | ||||||
| 100% | 72º(22) | 80º(27) | 91º(33) | 108º(42) | |||||||
| Apparent Temperature | Heat-stress risk with physical activity and/or prolonged exposure. |
| 90º-104º (32-40) | Heat cramps or Heat Exhaustion possible |
| 105º-130º (31-54) | Heat cramps or Heat Exhaustion likely. Heat Stroke possible. |
| 130º and up (54 and up) | Heat Stroke very likely. |
| Caution: This chart provides guidelines for assessing the potential severity of heat stress. Individual reactions to heat will vary. Heat illnesses can occur at lower temperature than indicated on this chart. Exposure to full sunshine can increase values up to 15º F. | |
Heat illnesses are the result of elevated body temperatures due to an
inability to dissipate the body's heat and/or a decreased fluid level. Always
remember that mild heat illnesses have the potential of becoming severe
life threatening emergencies if not treated properly (See Fluid Balance
above).
Heat cramps are a form of muscle cramp brought on by exertion and insufficient salt.
Replace salt and fluid (see Fluid Balance) and stretch the muscle (See
Chapter 6 - Wilderness Travel & Camping: Stretching). Kneading and pounding
the muscle is less effective than stretching and probably contributes to
residual soreness.
Heat Syncope (fainting) is a mild form of heat illness which results from physical exertion in a hot environment. In an effort to increase heat loss, the skin blood vessels dilate to such an extent that blood flow to the brain is reduced, resulting in symptoms of faintness, dizziness, headache, increased pulse rate, restlessness, nausea, vomiting, and possibly even a brief loss of consciousness. Inadequate fluid replacement which leads to dehydration contributes significantly to this problem.
Heat Syncope should be treated as fainting (See Fainting). The person should lie or sit down, preferably in the shade or in a cool environment. Elevate the feet and give fluids, particularly those containing salt (commercial "rehydration" mix or ½ teaspoon salt and ½ teaspoon baking soda per quart/0.9 liter) (see Fluid Balance page 00). The patient should not engage in vigorous activity for at least the rest of that day. Only after s/he has completely restored his/her body fluids and salt and has a normal urinary output should exercise in a hot environment be resumed (and then cautiously).
This occurs when fluid losses from sweating and respiration are greater than internal fluid reserves (volume depletion). Heat Exhaustion is really a form of volume shock. The lack of fluid causes the body to constrict blood vessels especially in the periphery (arms and legs). To understand Heat Exhaustion think of a car with a radiator leak pulling a trailer up a mountain pass. There is not enough fluid in the system to cool off the engine so the car overheats. Adding fluid solves the problem. The signs and symptoms of Heat Exhaustion are:
Victims of Heat Exhaustion must be properly re-hydrated and must be very
careful about resuming physical activity (it is best to see a physician
before doing so). Treatment is as described above for Heat Syncope, but
the person should be more conservative about resuming physical activity
to give the body a chance to recover. Have the person rest (lying down)
in the shade. Replace fluid with a water/salt solution (commercial "rehydration"
mix or ½ teaspoon salt and ½ teaspoon baking soda per quart/0.9
liter) (see Fluid Balance page 00). Drink slowly, drinking too much, too
fast very often causes nausea and vomiting.
Evacuation usually is not necessary. Heat Exhaustion can become
Heat Stroke if not properly treated (see Heat Stroke below). A victim
of Heat Exhaustion should have be closely monitored to make sure that their
temperature does not go above 103° F (39° C) If it does so, treat
the person for Heat Stroke as described below.
Heat Stroke is one of the few life threatening medical emergencies. A victim can die within minutes if not properly treated. Heat Stroke is caused by an increase in the body's core temperature. Core temperatures over 105° (41° C) can lead to death. The rate of onset of Heat Stroke depends on the individual's fluid status. To understand Heat Stroke think of that same car pulling a trailer up a mountain pass on a hot day. This time the radiator has plenty of fluid, but the heat challenge of the engine combined with the external temperature is too much. The engine can't great rid of the heat fast enough and the engine overheats. There are two types of Heat Stroke-fluid depleted (slow onset) and fluid intact (fast onset).
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