Heat stress

Combat Edge, July, 2004 by Monte Anderson

Heat stress is a combination of direct environmental variables (mainly temperature and humidity), work rate and clothing requirements. These factors combine with indirect acclimatization and physical conditions to increase body temperature and cardiovascular demands. Acclimatization to heat involves a series of physiological and psychological adjustments that occur in an individual during the first week of exposure to hot environmental conditions. Extra caution must be taken when workers who are not acclimated or physically fit must be exposed to heat stress conditions. The greater the heat stress is on these workers, the greater the resulting physiological strain. Heat stress can diminish performance and adversely affect health and safety. Most heat-related injuries can be avoided if people are aware of their environment and can recognize heat stress symptoms.

The three types of heat-induced illnesses include heat strain, heat exhaustion, and heatstroke. Transition from one to the next can be very evident, hardly noticeable, or not evident at all.

Heat strain is when the body temperature is between 99.5 and 100 degrees Fahrenheit. It reduces performance, dexterity, coordination, and alertness. Incidence and severity will vary among people.

Heat exhaustion is when the body temperature is between 101 and 104 degrees Fahrenheit. It may cause fatigue, nausea/vomiting, cramps, rapid shallow breathing, and fainting. The skin is pale, cool, clammy, and moist with profuse sweating, and the pulse rate is weak. In its most serious form, heat exhaustion leads to prostration and can cause serious injuries.

Heatstroke is when the body temperature is greater than 104 degrees Fahrenheit. It is the most serious heat-induced illness because of its potential to be life threatening or result in irreversible damage. Heatstroke results from the body losing its ability to lower its temperature. The heatstroke victim is often manic, disoriented, confused, delirious or unconscious. The victim's skin can be hot and dry because sweating has ceased. If treatment is not immediate, the victim's condition can deteriorate to convulsions, brain damage, and eventual death. Immediate emergency care and hospitalization are essential if signs of heatstroke develop. Cool down by any method available and transport to the nearest medical facility for treatment.

Assessment of heat stress can be conducted by measuring the physical factors of the environment. The commonly used area monitoring measurement is Wet Bulb Globe Temperature (WBGT). This index relates atmospheric effects to heat stress in outdoor and harsh industrial environments.

Physics of Heat

According to thermodynamics, and a great deal of paraphrasing, heat is transferred in three ways: radiation, convection, and conduction.

Radiation--This refers to heat that is exchanged from distant objects (like solar radiant heat or a heater in a room). Certain objects, depending on color and composition, retain and continue to radiate heat (like asphalt, rocks, or dark clothing).

Convection--This refers to the relative movement of air that increases heat exchange. This method is used in convection ovens to cook food efficiently and evenly. Air movement when the temperature exceeds body temperature may increase heat stress. Imagine again your convection oven. Fans cannot cool a person when the temperature exceeds 90 degrees Fahrenheit and humidity is greater than 35 percent. Fans have actually caused heat stress when the temperature is above 100 degress Fahrenheit. During outdoor activity, certain types of clothing limit the relative wind across the skin, restrict evaporation of sweat, and add a layer of trapped air as insulation.

Conduction--This refers to the direct contact with objects allowing heat exchange (i.e., touching a hot iron). Good thermal conductors transfer heat quickly. Sit on a hot metal playground slide wearing shorts and you will clearly understand conduction. Humidity is an environmental factor that makes it "feel hotter." As the relative humidity increases, evaporation is inhibited, reducing the effectiveness of the body's natural response to heat. This makes it feel much hotter than the actual temperature reflects. Meteorologically speaking, this is the heat index. The heat index, similar to windchill in the winter, establishes an apparent temperature by comparing either temperature/relative humidity (see heat index chart) or temperature/dew point.

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As physical activity increases, so does the amount of muscular heat produced in the body core. The more you move and exert yourself, the more heat your muscles produce. This is why one uncontrollably shivers during very cold temperatures, heat exhaustion, or even heatstroke, as the body can no longer maintain a safe core temperature.

Physiological Response to Heat

Skin plays a central role in maintaining a constant body temperature of 98.6 degrees Fahrenheit (37 C) in two ways. First, capillaries in the skin exchange heat with the environment. In hot weather, these capillaries dilate (cutaneous vasodilation), allowing increased blood flow and heat exchange along the surface of the skin through both conduction and convection. Besides sunburn, this is the reason skin appears red during exercise or hot weather. Second, the skin aids in cooling the core temperature through evaporation. When body temperature increases above 98.6 degrees Fahrenheit, the body actively secretes sweat, containing water and salt from sweat glands in the skin, increasing evaporation and heat loss. Similarly, the exchange across the capillaries of the lungs and the release of water vapor through respiration regulate body temperature.