Can You TAKE THE HEAT?

American Fitness, Nov, 1999 by Barbara Belmont

To reduce the risk of heat injury, physical activity in elevated temperatures should be accompanied by preventive measures such as proper fluid intake and readily available first aid.

Thermal stress during exercise can affect anyone. Regardless of age, fitness level, body type or area of residence, anyone can experience some form of heat stress while exercising. Would you be able to recognize the symptoms? What physical and environmental parameters of such a potentially serious condition should be avoided?

The first two indices are a high ambient temperature and a high relative humidity. Exercising in a hot environment causes a higher heart rate and reduced stroke volume. This is primarily due to two blood circulation factors:

* delivering oxygen to the working muscles

* delivering heated blood from the organs and muscles to the peripheral areas

The results of over-stressing this system include a decrease in performance, heat injury or both. Therefore, under these conditions, either the thermal stress or exercise level must be reduced. During high ambient temperatures, body heat cannot be sufficiently lost. Heat exchange by evaporation of moisture on the skin is also impaired further during humid windless weather. Perspiration alone does not cool the body--it is the evaporation of the moisture which carries heat off. Relative humidity is the most important factor in determining how well heat is lost by evaporation. Relative humidity is the amount of moisture in the air surrounding the body compared to the amount of moisture that can potentially be carried by the air. If humidity is high, the air is less able to support further moisture from the evaporation of perspiration. Figure 1, taken from the ACSM Resource Manual 2nd edition, is a guideline for low, moderate or high heat stress conditions. You should consider canceling events/exercise that fall under the conditions listed under "high" heat stress. If you choose not to cancel an event, it is vital to provide the necessary medical staff and equipment as well as proper hydration techniques. Instruments to measure wet bulb globe temperature are commercially available, as are various formulas in the sources listed.

(Figure 1)

WBGT Heat Stress Index(*)

Degrees (Fahrenheit)   Degrees (Celsius)   Level of heat dress
80-84                  26.5-28.2           Low
85-87                  29.5-30.5           Moderate
88                     31.2                High

Degr F = (9/5 x Degr C)   32

(*) Taken from ACSM Resource Manual for Guidelines for Exercise Testing and Prescription, 2nd edition, 1993.

In addition to ambient temperature and relative humidity, several other indices can increase the risk of heat injury. If you are leading an exercise group or event, it is essential to know at least a bit about your students so you can be aware of who may be at risk.

Heat stress can sneak up on you slowly. Body size and percentage of body fat can put a person at risk. Obesity increases the likelihood of heat stress. Aerobic conditioning, level of acclimatization and exercise intensity relative to one's maximum work capacity are also factors to consider. Restrictive, heavy, rubberized clothing adds weight and prevents evaporation of moisture and can be dangerous to exercise in, regardless of ambient temperature. Ground temperature is also important to consider. Ground temperature can add significantly to the ambient temperature, particularly late in the afternoon.

Most heat ailments can be prevented by the amount of fluid and electrolyte intake. Heat cramps normally occur in the abdomen or calves and are usually painful but fairly benign. They may be caused by a potassium/sodium imbalance across the muscle cell membranes. Moderate salting of food and adequate water consumption are often sufficient to resolve the problem. Sodium depletion can occur over a long exercise bout of several hours or a several-day event. Mild sodium depletion may cause dizziness or fatigue, and moderate to severe depletion can cause nausea, vomiting or cramping. Severe sodium depletion can cause seizures, coma or even death. Sodium depletion is not that common since most diets are usually adequate, but the risk increases with very long events, especially during heat exposure. During heavy training, prophylactic increases in salt intake will probably help, but no more than 1/2 teaspoon of salt per liter of water is necessary. The purpose is to maintain sodium concentrations which drive the thirst mechanism, and more sodium than the aforementioned is not palatable or been proven necessary.

Dehydration can be caused by failure to rehydrate on a regular basis. This can also significantly contribute to other heat stress injuries, particularly if the activity is continued. Early symptoms include decreased urine output, lethargy, anxiety and/or irritability. Severe dehydration can cause uncoordinated gait and spastic muscle movement as well as altered consciousness. This can lead to a cardiovascular collapse and ultimately death. Dehydration can lead to heat exhaustion, which can progress to heat stroke. Common symptoms a person may experience in progressing from dehydration to heat exhaustion would be "goose bumps" on the skin, dizziness, headache, shortness of breath, vomiting and/or uncoordinated gait. The body temperature is typically elevated but less than 103 [degrees] F. During an emergency, you should rehydrate the individual and take active cooling measures as described below.