The basics of heat stress

When the thermometer rises, it can-and often does-create a multitude of problems. Anyone, given the right (or wrong) conditions, can get heat stress. Some are lucky enough to suffer only from heat cramps, while those who are less fortunate may be laid up by heat exhaustion or devastated by heat stroke. As the long, hot days of summer approach, it is helpful to review the effects of warm weather on the human body, the illnesses that may result and what you can do.

How the body stays cool Unknowingly, you constantly engage your body in the life-and-death struggle to disperse the heat it produces. If allowed to accumulate, this heat would quickly increase your body temperature beyond its comfortable 98.6oF. This does not normally happen because your body is able to lose enough heat to maintain a steady temperature. You become aware of this struggle for heat balance during hard labor or exercise in hot environments, when your body produces heat faster than it can lose it. Under certain conditions, your body may build up too much heat, your temperature may rise to life-threatening levels, and you may become delirious or lose consciousness. This is called heat stroke, and it is a serious medical emergency. If you do not rid your body of excess heat fast enough, it cooks the brain and other vital organs. It often is fatal, and those who survive may have permanent damage to their vital organs. Before your temperature reaches heat-stroke levels, however, you may suffer heat exhaustion with its flu-like symptoms, and while treating its symptoms you avoid heat stroke.

How does your body dispose of excess heat? Humans lose heat largely through their skin, similar to how a car loses heat through its radiator. Exercising muscles warms the blood, just as a car's hot engine warms its radiator fluid. Warm blood travels through the skin's dilated blood vessels losing heat by evaporating sweat to the surrounding air, just like a car loses engine heat through its radiator.

When blood delivers heat to the skin, two of the most important ways the body loses heat are radiation and evaporation (vaporization of sweat). When the temperature is 70oF or less, the body releases its heat by radiation. As environmental temperatures approach your body temperature, you lose less heat through radiation. In fact, people working on hot summer days actually gain heat through radiation from the sun. This leaves evaporation as the only way to effectively control body temperature.

Water loss Your body is about half water. You lose about 2 quarts every day (breathing, urinating, bowel movements and sweat). A working adult can produce 2 quarts of sweat per hour for short periods and up to 15 quarts per day. Because the body's water absorption rate of 1.5 quarts per hour is less than the body's 2 quarts per hour sweat rate, dehydration results. This happens because you cannot drink enough water to keep up with your sweat losses.

If you drink only when you are thirsty, you are dehydrated already. Thirst is not a good guide for when to drink water. In fact, in hot and humid conditions, you may be so dehydrated by the time you become thirsty that you will have trouble catching up with your fluid losses. One guideline regarding your water intake is to monitor your urine. You are getting enough water if you produce clear urine at least five times a day. Cloudy or dark urine, or urinating less than five times a day, means you should drink more.

In the Gulf War, American armed forces followed the practice of the Israeli army: drinking a minimum of 1 quart of fluid per hour. This tactic resulted in zero deaths from heat illness. In contrast, during the Six Day War of 1967, more than 20,000 Egyptian soldiers died3/4with no visible wounds3/4most likely from dehydration and heat illness because they were restricted to 3 quarts daily.

While working in hot weather, drink 8 ounces of water every 20 minutes. Generally, 16 ounces is the most a person can comfortably drink at once. You cannot "catch up" by drinking extra water later because only about 1 quart of water per hour can pass out of the stomach. Therefore, if possible, workers should begin drinking water before they start work.

Cool water (50oF) is easier for the stomach to absorb than warm water, and a little flavoring may make the water more tasty. The best fluids are those that leave the stomach fast and contain little sodium and some sugar (less than 8 percent). You should avoid coffee and tea because they contain caffeine, which is a diuretic that increases water loss through urination. Alcoholic beverages also dehydrate by increasing urination. Soda pop contains about 10 percent sugar and, therefore, your body does not absorb it as well as water or commercial sports drinks. The sugar content of fruit juices ranges from 11 to 18 percent and has an even longer absorption time. Commercial sports drinks contain about 5 to 8 percent sugar.

Electrolyte loss Sweat and urine contain potassium and sodium, which are essential electrolytes that control the movement of water in and out of the body's cells. Many everyday foods contain these electrolytes. Bananas and nuts are rich with potassium, and most American diets have up to 10 times as much sodium as the body needs. Getting enough salt is rarely a problem in the typical American diet. In fact, most Americans consume an excessive amount of sodium-averaging 5 to 10 grams of sodium per day-although we probably require only 1 to 3 grams. Therefore, sodium loss is seldom a problem, unless a person is sweating profusely for long periods and drinking large amounts of water.

Commercial sports drinks can be useful if you are participating in vigorous physical activity for longer than 1 hour (some experts say longer than 4 hours). Most of the time, however, people merely require water to remain hydrated. The truth is that excessive sodium can draw water out of the body cells, accentuating the dehydration. In addition, drinking large amounts of water (more than 1 quart an hour) can cause water intoxication, a condition that flushes electrolytes from the body. Frequent urination and behavior changes (irrationality, combativeness, coma, seizures, etc.) are signs of water intoxication.

Effects of humidity Sweat can only cool the body if it evaporates. In dry air, you will not notice sweat evaporating. However, sweat cannot evaporate in high-humidity conditions; it just drips off the skin. At about 70-percent humidity, sweating is ineffective in cooling the body.

Because humidity can significantly reduce evaporative cooling, a highly humid but mildly warm day can be more stressful than a hot, dry one. Therefore, the higher the humidity, the lower the temperature at which heat risk begins, especially those who are generating heat with vigorous work.

Who is at risk? Everyone is susceptible to heat illness if environmental conditions overwhelm the body's temperature-regulating mechanisms. Heat waves can set the stage for a rash of heat-stroke victims. For example, during the 1995 summer heat wave in Chicago, the death toll reached 590.

People who are obese, chronically ill or alcoholics have an increased risk. The elderly are at higher risk because of impaired cardiac output and decreased ability to sweat. Infants and young children also are susceptible to heat stroke, as well.

The fluid loss and dehydration resulting from physical activity puts outdoor laborers at particular risk. Certain medications predispose individuals to heat stroke, such as drugs that alter sweat production (antihistamines, antipsychotics, antidepressants) or interfere with thermoregulation.

Heat illnesses Several disorders exist along the spectrum of heat illnesses. Heat cramps, heat exhaustion and heat stroke are on the more serious side of the scale, whereas heat syncope, heat edema and prickly heat are less serious (see "Heat illnesses," page C 18). Only heat stroke is life-threatening. Untreated heat-stroke victims always die.

* Heat cramps are painful muscular spasms that occur suddenly. They usually involve the muscles in the back of the leg or the abdominal muscles. They tend to occur immediately after exertion and are caused by salt depletion. Victims may be drinking water without adequate salt content. However, some experts disagree because the typical American diet is heavy with salt.

* Heat exhaustion is characterized by heavy perspiration with normal or slightly above-normal body temperatures. A depletion of water or salt3/4or both3/4causes this condition. Some experts believe severe dehydration is a better term because it happens to workers who do not drink enough fluids while working in hot environments. Symptoms include severe thirst, fatigue, headache, nausea, vomiting and diarrhea. The affected person often mistakenly believes he or she has the flu. Uncontrolled heat exhaustion can evolve into heat stroke.

* Heat stroke is classified in two ways: classic and exertional. Classic heat stroke, also known as the "slow cooker," may take days to develop. This condition is prevalent during summer heat waves and typically affects poor, elderly, chronically ill, alcoholic or obese persons. Because the elderly often have medical problems, heat stroke exacerbates the problem, and more than 50 percent of elderly heat-stroke victims die3/4even with medical care. Death results from a combination of a hot environment and dehydration. Exertional heat stroke also is more common in the summer. You see it frequently in athletes, laborers and military personnel who sweat profusely. Known as the "fast cooker," this condition affects healthy, active individuals who strenuously work or play in a warm environment. Exertional heat-stroke victims usually are sweating when stricken, while the classic victims are not sweating. Its rapid onset does not allow enough time for severe dehydration to occur.

Because uncontrolled heat exhaustion can evolve into heat stroke, you should know how to tell the difference between them. If the victim feels extremely hot when touched, suspect heat stroke. Another mark of heat stroke is that the victim's mental status (behavior) changes drastically3/4ranging from being slightly confused and disoriented to falling into a coma. In between these conditions, victims usually become irrational, agitated or even aggressive and may have seizures. In severe cases, the victim can go into a coma in less than 1 hour. The longer a coma lasts, the lower the chance for survival, so rescuers must be quick.

A third way of distinguishing heat stroke from heat exhaustion is by rectal temperature. Obviously, this is not very practical because conscious heat-stroke victims may not cooperate. Taking a rectal temperature can be embarrassing to both victim and rescuer. Moreover, rectal thermometers are seldom available, and the whole procedure of finding the appropriate thermometer and then using it wastes time and distracts from important emergency care. In most cases, an ambulance arrives within 10 to 20 minutes.

* Heat syncope, in which a person becomes dizzy or faints after exposure to high temperatures, is a self-limiting condition. Victims should lie down in a cool place when it occurs. Victims who are not nauseated can drink water.

* Heat edema, which is also a self-limiting condition, causes ankles and feet to swell from heat exposure. It is more common in women unacclimated to a hot climate. It is related to salt and water retention and tends to disappear after acclimation. Wearing support stockings and elevating the legs often helps reduce swelling.

* Prickly heat, also known as a heat rash, is an itchy rash that develops on skin that is wet from sweating. Dry and cool the skin.

Cooling methods Sometimes the only way to stop possible damage is to cool the victim as quickly as possible. However, it is important to pay attention to both the cooling methods and cautions.

* Ice baths cool a victim quickly but require a great deal of ice3/4at least 80 pounds3/4to be effective. Needing a big enough tub also limits this method. Cool-water baths3/4(less than 60oF)3/4can be successful if you stir the water to prevent a warm layer from forming around the body. This is the most effective method in highly humid conditions (greater than 75-percent humidity).

* Spraying the victim with water combined with fanning is another method for cooling the body. The water droplets act as artificial sweat and cool the body through evaporation. However, this method is not effective in high humidity3/4greater than 75 percent.

* Ice bags wrapped in wet towels and placed against the large veins in the groin, armpits and sides of the neck also cool the body, though not nearly as quickly as immersion.

Cautions to remember when employing any cooling method include: * Do not delay the onset of cooling while waiting for an ambulance. Doing so increases the risk of tissue damage and prolonged hospitalization. * Stop cooling when the victim's mental status improves to avoid hypothermia. * Do not use rubbing alcohol to cool the skin. It can be absorbed into the blood, causing alcohol poisoning. Its vapors are a potential fire hazard. * Do not use aspirin or acetaminophen. They are not effective because the brain's control-center temperature is not elevated as it is with fever caused by diseases.

Adjusting to heat Most heat illness occur during the first days of working in the heat. Therefore, acclimation (adjusting to the heat) is the main preventive measure. To better handle the heat, the body adjusts by decreasing the salt content in sweat and increases the sweating rate. Year-round exercise can help workers prepare for hot weather. Such activity raises the body's core temperature so it becomes accustomed to heat. Full acclimation, however, requires exercise in hot weather. You can do this by exercising a minimum of 60 to 90 minutes in the heat each day for 1 to 2 weeks.

The acclimated heart pumps more blood with each stroke than a heart unused to working in the heat. Sweating earlier and doubles the amount of sweat per hour from 1.5 quarts to 3 quarts or more.

When new workers are exposed to hot weather, team them with veterans of the heat who know how much water to drink. Heat illnesses are avoidable. With knowledge, preparation, fluid replacement and prompt emergency care, heat casualties need not be a factor for those working in warm weather.

Dr. Alton Thygerson is a professor of health science at Brigham Young University, Provo, Utah. He also serves as the technical consultant for the National Safety Council's First Aid Institute.

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