The Heat is On!
Larry Birnbaum, PhD, EPC
Associate Professor
Department of Exercise Physiology
The College of St. Scholastica
Duluth, MN  55811

Here in the Northland we are sweltering in one of the hottest summers on record.  We should not complain though because most of the country is experiencing the same and most places are hotter than us.  The up side to this prolonged heat wave is that we have seen sunshine just about every day this summer.  I can’t remember the last cloudy day we had.  That is unusual for us and welcomed by the sun worshippers.  So, what does this heat have to do with exercise physiology?

Heat stress is part of the domain of exercise physiologists.  We have the expertise that we should share with the general public during heat waves.  We could write articles for local newspapers, do short interviews for radio and television, and of course, provide information to our clients in cardiac rehabilitation, fitness facilities, and so forth.  What information should we share with the public?  We want to provide sufficient information so that people will make appropriate decisions about working or exercising in heat, but we don’t want to lose the message in details.  Three areas should be covered.  First, a brief overview of how the body responds to heat will help people understand the other two areas that should be covered, namely symptoms associated with heat stress and precautions that should be taken to avoid heat stress.

Everyone knows that the body perspires to help it lose heat through evaporative cooling.  The effectiveness of sweating for cooling the body depends on the air temperature, humidity, and wind speed.  Generally, the hotter the air, the faster sweat evaporates.  Wind also facilitates evaporation.  At high temperatures, relative humidity becomes the most important factor (1).  As humidity increases, the evaporation rate declines.  Thus, a person could be sweating profusely in a hot, humid, windless environment and achieve very little, if any, cooling effect.  Indeed, these are the most dangerous conditions for developing hyperthermia and dehydration.

The body can also lose heat via radiation, convection, and conduction if the air temperature is less than the skin temperature (1).  Unfortunately, these mechanisms lead to heat gain when the environmental temperature exceeds skin temperature.  As such, the body has to rely on perspiration to lose heat.

Dehydration and hyperthermia are the two primary concerns associated with heat stress.  As the body loses water through perspiration (and small amounts in exhaled breath), blood volume decreases, which leads to decreased stroke volume and decreased cardiac output (2).  Working in a hot environment creates a dilemma for the cardiovascular system as it tries to deliver sufficient blood to the working muscles to meet oxygen needs and to the skin to facilitate radiative and evaporative heat loss (1,2).  A decrease in blood volume exacerbates the problem.  A reduced capacity for heat loss sets the stage for hyperthermia.  It should be noted that hyperthermia can occur in the absence of severe dehydration (2).  As the core body temperature increases, central nervous system function is adversely affected.

The symptoms associated with heat stress can be categorized into three heat-related disorders, heat cramps, heat exhaustion, and heat stroke (1,2).  Heat cramps, the least serious of the disorders, is characterized by severe cramping of skeletal muscles probably due to mineral losses and dehydration.  When we perspire, we not only lose water but also electrolytes and minerals such as sodium, chloride, potassium, calcium, and magnesium.  The sweat glands can reabsorb some sodium and chloride, but not potassium, calcium or magnesium.  Exercise physiologists know that these ions are essential for proper muscle function.  Heat cramps may be treated by moving the individual to a cooler location and administering fluids.

The symptoms of heat exhaustion include extreme fatigue, breathlessness, dizziness, vomiting, fainting, cold and clammy or hot and dry skin, hypotension, a weak and rapid pulse.  The cardiovascular system cannot meet the body’s needs as the skin and muscles are competing for cardiac output.  The symptoms are due to decreased blood volume.  Thermoregulatory mechanisms are functioning but cannot dissipate heat quickly enough.  This condition is generally not accompanied by a high rectal temperature (internal temp <39°C/102.2°F) (1).  Treatment includes moving the subject to a cooler location, elevating the feet to avoid shock, and giving salt water.

The most serious heat illness is heat stroke.  It is life threatening and requires immediate medical attention.  The thermoregulatory mechanisms fail.  It is characterized by an increased internal body temperature (>40°C/104°F), cessation of sweating, hot, dry skin, a rapid pulse, increased respiration, hypertension (usually), and confusion or unconsciousness.  It will progress to coma and death if untreated.  High internal temperatures can result in permanent brain damage.  It is treated by rapidly cooling the body in cold water or an ice bath.

The precautions for avoiding heat stress, specifically hyperthermia, are straight forward and seem to be a matter of common sense.  Part of the problem is that people do not realize when they are starting to get into trouble.  They may not be aware of how much water they are losing and may not recognize warning signs.  Thus, recognizing the symptoms of hyperthermia is one the most important precautions.  Symptoms associated with heat cramps and heat exhaustion should be strong warning signs to take immediate corrective action to prevent heat exhaustion.  As the body temperature rises, subjective symptoms include a cold sensation over stomach and back with piloerection (goose bumps) when the rectal temperature is 40-40.5 °C (104-105 °F), progressing to muscle weakness, disorientation, and loss of postural equilibrium at a rectal temperature of 40.5-41.1 °C (105-106 °F), and finally diminished sweating, loss of consciousness and hypothalamic control when the rectal temperature reaches 41.1-41.7 °C (106-107 °F) (1). 

Whenever persons are working or exercising in a hot environment, they should reduce their effort in both intensity and duration.  Take frequent breaks preferably in a cooler environment and drink lots of fluids, especially water.  Fluid intake should be increased before, during, and after the work or exercise bout.  Water loss has to be compensated for with increased water intake.  For the average individual, sports drinks are probably not necessary.  Fluid and electrolyte (salt) loss can be balanced with increased water intake and a normal diet.  Sports drinks combined with water may be beneficial if sweating is profuse and prolonged.  If possible do not work during the hottest part of the day and wear light colored and loosely woven clothes made of fabric that wicks moisture away from the skin.  Cotton is far more comfortable that polyester, but neither fabric wicks moisture away from the skin.  Long fiber wool and several synthetic fabrics are available that wick moisture away from the skin.

Obese persons are more susceptible to heat stress because adipose tissue insulates the body reducing heat loss.  Children are also at an increased risk due to their smaller blood volume.  They dehydrate faster.  Good physical condition helps to decrease the risk to heat stress.  The physiological adaptations improve cardiovascular function and heat loss mechanisms.

During 1979-2002, 4,780 deaths were classified as heat related because of weather conditions (3).  Between 1998 and 2006, there were 303 hyperthermia deaths of children left in cars in the United States (4).  While these numbers pale in comparison to mortality associated with heart disease and cancer, the deaths are preventable.  Exercise physiologists can play a vital role is helping to prevent heat-related morbidity and mortality by educating the public through channels mentioned above.

References

1.  Willmore, J. H. and Costill, D. L. (2004). Physiology of Sport and Exercise.   Champaign, IL:  Human Kinetics.

2.  Robergs, R. A. and Roberts, S. O. (1997). Exercise Physiology:  Exercise, Performance, and Clinical Applications.  St. Louis:  Mosby.

3. Centers for Disease Control. (2005). Morbidity and Mortality Weekly Reports. 54:628-630 [Online]. Retrieved July 26, 2006, from http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5425a2.htm.

4. Null, J. (2006). Hyperthermia Deaths of Children in Vehicles. Department of Geosciences. [Online]. Retrieved July 26, 2006, from http://ggweather.com/heat/.