Athletic participation in warm climates risks heat illness, from the mild and easily treated condition of heat stress to life threatening heat stroke.  In between, lie the conditions of heat cramps and heat exhaustion.  Each is discussed in some detail further on, but we need to first understand normal cooling mechanisms and their limitations. 

There are four general methods of heat loss: radiation, conduction, convection, and evaporation.  The first three are largely passive, although active physiologic processes, such as flushing, can enhance their effectiveness.  However, all three depend upon Newton’s Law of Cooling, which states that heat transfers at a rate proportional to the difference between an object’s temperature and that of its environment.  As normal human body temperature is 37° C (98.6° F), when the ambient temperature is 35° C (95° F) the temperature difference between the environment and the body is so little that these three passive mechanisms cease to effectively allow cooling.  In fact, as environmental temperatures rise further, these passive mechanisms may actually cause heat gain instead of heat loss. 

To combat such high temperature, the human body relies on evaporative heat loss, mainly through sweating.  The energy required to evaporate water (i.e., convert it from its liquid form into its vapor form) is considerable.  Because heat energy from the underlying skin is used for this process, the result is heat loss from the body.  The active process of sweating produces water beads on the skin so that the evaporative heat loss process can work.  This requires a little energy and potentially significant amounts of body water.  Some body salts, also referred to as electrolytes, are required as well, due to the fact that sweat glands in the skin are not able to produce completely salt-free sweat.  So in order to keep cool, the body expends water, some salt, and a little energy, all of which are not unlimited and therefore need replenishment.

When the relative humidity increases to 75%, however, even evaporative heat loss becomes inefficient because sweat has more difficulty everything into air already saturated with water vapor.  Moreover, the ability of sweating to cool the body becomes very poor as humidity exceeds 90%.  Given the above, there is really no effective natural way to cool in temperatures exceeding 35° C (95° F) with 90% humidity or higher.  Prolonged strenuous activity in such conditions strongly risks dangerous elevations in body temperatures.  This can lead to disordered metabolism as the thousands of complex chemical reactions occurring within the human body need to be tightly regulated and increased body temperature negatively effects them occurring at the normal physiologic rate, leading to metabolic havoc.

The resultant alteration in metabolism has been termed heat illness, which can be divided into four progressive stages.  These stages should not be considered absolute, as considerable overlap can occur from one stage to the next, and some stages may be very transient in a given individual, or skipped altogether.  Nevertheless, these four stages are helpful in determining the severity and proper treatment of heat illness.

The first stage of heat illness is termed heat stress.  In this stage, core body temperature remains normal and mild symptoms of heat edema or heat rash may be present, as may be generalized symptoms of fatigue or dizziness.  The heart rate tends to be higher than normal.  Oral intake of fluids usually provides effective treatment.

The next stage of heat illness is called heat cramps.  Although core body temperature remains normal, significant loss of body water and salt results in muscle cramping, nausea, or even vomiting, in addition to the symptoms and signs seen in heat stress.  Oral intake of fluid with electrolytes is usually sufficient although intravenous fluids may be required, especially if oral intake is not possible due to vomiting. 

The third stage of heat illness has been termed heat exhaustion.  Core body temperature in this condition is often elevated, although not above 40° C (104° F).  Syncope (i.e., fainting) and orthostasis (i.e., feeling faint or light-headed due to a drop in blood pressure when getting to an upright posture) can occur, as can headaches and emotionally irritability.  Hyperventilation and fast heartbeat are common.  There is usually significant water and salt loss accompanying this stage, requiring aggressive replenishment, either orally or intravenously, depending upon the severity of the condition and the ability to take large amounts of fluid orally.  Unlike the previous two stages, continued athletic participation is not recommended for those suffering from heat exhaustion, even after adequate treatment and replenishment. 

The most serious stage of heat illness is heat stroke.  With this, the core body temperature exceeds 40° C (104° F) accompanied by mental status changes such as impaired judgment, delirium, and ataxia or even seizures or coma.  Rhabdomyolysis (the acute breakdown of muscle tissue) can occur as can failure of vital organs such as the kidneys and liver.  Diarrhea may occur along with pulmonary edema (i.e., fluid in the lungs) making it difficult to keep blood oxygenated, resulting in potentially fatal acute respiratory distress syndrome (ARDS).  Cerebral edema (potentially fatal swelling of the brain) may also occur as may a condition called disseminated intravascular coagulation (DIC),  in which the normal clotting of blood is altered to the point that spontaneous clotting may occur in some blood vessels and at the same time other vessels may profusely bleed.  There may be a precipitous drop in blood pressure, as well as electrolyte imbalances, ischemia of the pancreas or intestines, and cardiac muscle injury or arrhythmias.  Death often results if the core body temperature exceeds 42° C (107.6° F), even with optimal treatment.  Early recognition is crucial for effective treatment for this severe form of heat illness, in which there is failure of body thermal regulation, a process normally controlled by a part of the brain known as the anterior hypothalamus and effected through the involuntary autonomic nervous system.  The classic presentation of hot, dry skin may not be present in cases of exertional heat stroke.  Treatment includes intravenous fluids for rehydration and replenishment of electrolytes as well as cooling with ice packs or an ice bath and administration of oxygen.  Emergent transportation to a medical facility for close observation should also be contemplated.  All athletic endeavors should be avoided until full recovery, which may take days, possibly longer.

Obviously, then, heat illness is best prevented.  Avoiding dehydration is key, as is avoiding salt depletion.  Thirst is of some benefit as its presence indicates need for fluid intake, but it is not a reliable indicator of hydration and often manifests late, after significant dehydration has already occurred.  Plain water is adequate to replenish what is lost from sweating from relatively short bouts of exertion.  For activities exceeding 1 hour, especially if accompanied by significant sweating, salt replacement via tablets or sports drinks is more optimal.  Lack of adequate salt replacement in the face of adequate water replacement can lead to hyponatremia (low blood sodium concentration) resulting in seizures and even permanent brain damage.

A difficult to quantify modifier to the development of heat illness is acclimation to high temperature.  But this takes time, typically days to weeks.  Also, some individuals may be more resistant to heat illness than others, possibly due to genetic factors (e.g., race) and/or lifestyle (e.g., fasting).  How much these factors affect one’s ability to avoid heat illness, if any, is not clear given the current lack of research.  Those who have had previous heat illness, however, appear to be more susceptible to subsequent bouts.

Furthermore, substances such as stimulants (possibly including even caffeine), and medicines containing decongestants or anticholinergics can negatively affect the autonomic nervous system’s capacity to regulate body heat.  Also, medical conditions such as obesity may predispose to heat illness, as can cystic fibrosis, due to persons with is condition producing sweat containing much more salt than usual, thereby requiring earlier and more significant salt replenishment.

To help understand the synergistic effect of elevated environmental temperature and humidity on the human body, the heat stress index was created.  The table given below is from the United States National Weather Service.  Care should be taken for prolonged exposure or physical activity when the heat stress index value is above 90, and especially when above 95.