In hot weather, especially with high 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 heatstroke. 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 the heat challenge is greater than heat loss (positive factors), you are at risk for a heat-related injury. In order to reduce the risk, you need to either decrease the heat challenge or increase your heat loss. Maintaining proper fluid balance is a central part of exercising in a heat challenge.

Heat Challenge—Negative Factors
* Temperature
* Exercise
* Humidity (see Heat Index below)
* Body wetness from sweating, rain, or water
* Wind (see Windchill Index)

Passive Heat Loss—Positive Factors
* Body size/shape—your surface-to-volume ratio affects how quickly you lose heat
* Insulation—type of clothing affects how you lose heat
* Body fat—amount of body fat affects how quickly you lose heat
* Shell/Core Response—allows the body shell to act as a thermal barrier

Active Heat Loss—Positive Factors
* Radiant heat from the body
* Sweating—ability to sweat is limited by fluid levels and level of fitness
* Wind (see Windchill Index)

The Heat Index

Ambient temperature is not the only factor 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 evaporate sweat and cool the body, increasing the risk of heat illnesses. The Heat Index shows the relative effects of temperature and humidity.

Heat Index

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 15° 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 life-threatening emergencies if not treated properly.

Heat Cramps

Heat cramps are a form of muscle cramp brought on by exertion and insufficient salt.

Treatment

Replace salt and fluid and stretch the muscle. Kneading and pounding the muscle is less effective than stretching and probably contributes to residual soreness.

Heat Syncope

Heat syncope (fainting) is a mild form of heat illness that results from physical exertion in a hot environment. In an effort to increase heat loss, the blood vessels in the skin 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 that leads to dehydration contributes significantly to this problem.

Treatment

Heat syncope should be treated as 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 1/2 teaspoon salt and 1/2 teaspoon baking soda per quart or liter). The patient should not engage in vigorous activity at least for the rest of that day. Only after she has completely restored her body fluids and salt and has a normal urinary output should exercise in a hot environment be resumed, and then cautiously.

Heat Exhaustion

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 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:

* Sweating
* Skin—pale, clammy (from peripheral vasoconstriction)
* Pulse rate increased
* Respiration rate increased
* Temperature normal or slightly elevated
* Urine output decreased
* Patient feels weak, dizzy, thirsty, “sick,” anxious
* Nausea and vomiting (from decreased circulation in the stomach)

Treatment

Victims of heat exhaustion must be properly rehydrated and must be very careful about resuming physical activity (it is best to see a physician before doing so). Treatment is as described 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 1/2 teaspoon salt and 1/2 teaspoon baking soda per quart or liter). Drink slowly; drinking too much, too fast very often causes nausea and vomiting.

Evacuation is not usually necessary. Heat exhaustion can become heatstroke if not properly treated. A victim of heat exhaustion should be closely monitored to make sure that her temperature does not go above 103° F (39° C). If it does, treat the person for heatstroke.

Heatstroke

Heatstroke is an immediate life-threatening medical emergency. A victim can die within minutes if not properly treated. Heatstroke is caused by an increase in the body’s core temperature. Core temperatures over 105° F (41° C) can lead to death. The rate of onset of heatstroke depends on the individual’s fluid status. To understand heatstroke, 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 get rid of the heat fast enough and the engine overheats.

There are two types of heatstroke—fluid depleted (slow onset) and fluid intact (fast onset).

* Fluid depleted: The person has heat exhaustion due to fluid loss from sweating and/or inadequate fluid replacement, but continues to function in a heat challenge situation. Ultimately, the lack of fluid minimizes the body’s active heat-loss capabilities to such an extent that the internal core temperature begins to rise. Example: a cyclist on a hot day with limited water.
* Fluid intact: The person is under an extreme heat challenge. The heat challenge overwhelms the body’s active heat-loss mechanisms even though the fluid level is sufficient. This typically has a very fast onset. Example: a cyclist pushing hard on a 104° F day (40° C).

Signs and Symptoms of Heatstroke

The key to identifying heatstroke is hot skin. Some victims may have hot dry skin; others may have hot wet skin because they have just moved from heat exhaustion to heatstroke. Also look for:

* Pale skin (Peripheral vasoconstriction)
* Increased pulse rate
* Increased respiratory rate
* Decreased urine output
* Increased temperature (may be over 105° F/41° C)—skin hot to the touch
* Skin that is wet or dry and flushed
* Severe changes in mental status and motor/sensory changes; the person may become comatose; possibility of seizures
* Pupils that are dilated and unresponsive to light

Treatment

Efforts to reduce body temperature must begin immediately! Move the patient (gently) to a cooler spot or shade the victim. Remove clothing. Pour water on the extremities and fan the person to increase air circulation and evaporation, or cover the extremities with cool wet cloths and fan the patient. Immersion in cool (not cold) water is also useful. Cooling extremities should be massaged vigorously to help propel the cooled blood back into the core.

After the temperature has been reduced to 102° F (39° C), active cooling should be reduced to avoid hypothermia (if shivering begins, it produces more heat). The patient must be monitored closely to make sure her temperature does not increase again. She will probably need fluids regardless of the type of onset. Apply basic life support (CPR) if needed. Afterward, there can be serious medical problems. Prepare to evacuate your patient.

From Teresa: I don’t normally copy web content, but this is so well-written and as I am not an expert in the medical field, I chose to do so for this article. Many thanks again to the Princeton Outdoor Action which took material from The Backpacker’s Field Manual.