Heat Illnesses

Our bodies have four main mechanisms through which we loose heat: evaporation, conduction, radiation and convection. Evaporation occurs when sweat is produced and evaporates off the skin causing a cooling effect. Conduction occurs when heat is transferred from a warmer object (person) to a cooler object (floor). This also cools the body but not as much as sweating and evaporation. Radiant heat loss occurs all the time but efficacy decreases as the outside temperature and humidity increases up to a temperature of 99 °F, the point at which heat transfer reverses direction (from surrounding air to body). Convection is the removal of heat from the skin to the surrounding air as air blows by (this is how the weather service determines heat index and wind chill). These four mechanisms as well as environmental and individual factors all contribute to maintaining temperature balance in the body.

Hundreds of people die each year from heat stroke and most of these deaths could have been prevented. People at high risk should be aware of their risk and on very hot days and avoid being outside or participating in heavy activities. People are at high risk for various reasons, medical conditions, medications and illicit drugs and strenuous activity in very hot and humid conditions. Medical conditions can predispose a person to heat illness by inhibiting sweat production and evaporation, conditions like obesity, generalized skin diseases, decreased blood flow to skin, dehydration, malnutrition, hyperthyroidism, cystic fibrosis, low blood pressure and heart failure. Medications can also impair sweating, drugs like atropine, Benadryl, tricyclic antidepressants, MAO inhibitors, Dexedrine, pseudofed, Beta-blockers and many more. Illicit drugs like PCP, LSD, speed and cocaine can all cause increased muscle activity which generates more body heat. Drug withdrawal syndromes and prolonged seizures may have the same effect. The risk of heat illness also increases with age, concurrent illness and reduced physical fitness. Even extremely physically fit individuals are at risk when participating in strenuous outdoor activity when the temperature and humidity are very high.

People who are going to be exposed to extreme temperatures over long periods of time should acclimate to the temperature slowly. This is achieved by scheduled exposures to the heat and gradually increasing the duration of exposure and workload. On average, it takes about 100 minutes of exposure a day for 2-3 weeks to become acclimatized.

There are four main medical illnesses that can result from excessive exposure to hot environments: heat syncope (fainting), heat cramps, heat exhaustion and heat stroke, the most severe illness.

Heat syncope or a sudden fainting episode can result from overheating. The skin tries to loose heat more efficiently by opening up or widening the blood vessels and causing a decreased blood flow to the brain and other vital organs. The lack of blood flow to the brain causes the fainting episode, which is usually very quick. A person who experiences heat syncope will have a fast but weak pulse and cool, moist skin. The most important thing to do is loosen all tight clothing, put the person in a cool environment and have them take fluids by mouth to rehydrate their body. If the person remains unconscious emergency services are needed immediately.

Heat cramps are uncontrollable contractions or even spasms of muscles, usually the larger muscles like the legs, which can last seconds to minutes. The cramps occur because of salt and water depletion from sweating which is being replaced by water alone. The person is alert with cool, moist skin and may be agitated and complain of pain. The person affected should move into a cool environment, by themselves or with assistance, and replace the lost salt and water. Mixing some salt into a liter of water and drinking it is usually sufficient.

Heat exhaustion is a precursor to heat stroke and must be taken seriously. It results from dehydration (depletion of salt and water from excessive sweating without replacement). The person has a fast, weak pulse, cool, moist skin and may show symptoms of heat syncope and cramps. The victim may also be thirsty and weak, have a headache, impaired judgement, nausea, vomiting, increased rate of breathing and an elevated body temperature. The victim should be moved to a cool environment and replace the lost salt and water (1-2 liters of salt water over a period of 2-4 hours). It is recommended that the person rest, after an episode of heat exhaustion, for at least a day or two before returning to work or strenuous activity.

Heat stroke is a life threatening illness that requires immediate medical attention at the closest emergency department. It results from the failure of the four mechanisms that our bodies use for heat loss (see above). The victim will have impaired judgement (from confusion to coma) and high fever (typically over 105 °F) sometimes accompanied by a lack of sweating. The person might be dizzy and weak, have nausea, vomiting, diarrhea, blurred vision, increased rate of breathing and irrational behavior. The victims pulse is strong initially but rapidly gets weaker and faster. Persons at greatest risk for heat stroke are the very young, elderly (over 65), chronically ill and those on medications which interfere with the heat loss mechanisms. Heat stroke has serious complications and can result in death.

The most important thing to remember is that when the temperature and humidity are high, no one is immune to heat illnesses and they can be prevented. Recognizing the signs of heat illnesses will allow for early treatment and decrease the number of fatalities.