Firefighters at Risk for Heat Stress
by Shirley Pye
Thirty firefighters from FDNY were overcome by this summer's excessive heat and humidity while fighting a three-alarm fire in Brooklyn. The firefighters were treated at Kings County Hospital for heat exhaustion. Another five firefighters were treated for minor burns. in Operations involving high temperatures, high humidity, close proximity or direct physical contact or with hot objects, or strenuous physical activities have a high potential for causing heat injuries, also known as heat stress. Fire fighters, hazmat team members and other emergency responders as well as outside laborers who work under the hot sun, and those who work in occupations where heat is generated are all at risk of injury. This risk is increased by the wearing of protective clothing, especially chemical protective clothing.

Emergency Film Group's program Heat! Working in Hot Environments provides valuable safety training about avoiding heat injuries while working in protective clothing.

It is helpful to understand how the human body reacts to heat. The body’s internal core temperature – the temperature of the body’s deep tissues – must be maintained within a fairly rigid range from about 97 - 99.5oF. In an effort to maintain that core temperature, the body attempts to strike a balance between the amount of heat produced internally through the work of muscles and physiological mechanisms, and the amount of heat lost to the outside environment. This is accomplished through:

· Vasodilation: Blood vessels in the skin dilate, resulting in the transfer of the heat to the skin. This appears as red, warm skin;

· Sweat: When exposed to severe heat, blood flow to the skin increases, which causes the skin to sweat. As sweat evaporates, the skin is cooled. As a worker exerts more effort, the body cranks up the heat exchange system and will sweat more;

· Decreased heat production: Mechanisms that cause excess heat production, such as shivering and chemical thermogenesis (this occurs slowly).

The internal core temperature stays within safe ranges as long as this heat/cooling mechanism functions properly.

But if the environment is very humid, or if the heat is excessive, the heat exchange process can break down. Sweat evaporation slows or stops. The body can’t release excess heat, so heat is stored, causing the core temperature to rise. This puts the worker at risk for heat injury, also known as heat stress.

According to the American Conference of Industrial Hygienists (ACGIH), heat stress is defined as the net (overall) heat burden on the body from the combination of the body heat generated while working, environmental sources (air temperature, and humidity.) Other factors that can contribute to the heat burden are air movement, radiation from the sun or hot surfaces/sources) and clothing. An early symptom of mild heat stress may be heat cramps, caused by dehydration. If steps are not taken to protect the body, heat exhaustion may quickly follow, characterized by fatigue and weakness, headache, nausea and vomiting, vertigo and giddiness, thirst, muscle cramps, profuse increased sweating, and fainting.

Heat stroke is the most serious form of heat stress, occurring when the body temperature reaches 106oF. Sweating may stop, making the body unable to rid itself of excess heat. Signs include mental confusion, delirium, loss of consciousness, convulsions or coma. Unless treated promptly, victims of heat stroke will die. Prompt first aid and transport to a medical facility can prevent permanent injury to the brain and other vital organs.

The Problem
Although people react differently to heat, even mild heat discomfort can create problems for most people. Emergency responders and other workers who are hot are less productive and less cautious about safety, and may affect the safety of an entire work group. Athletes who must practice or play in the heat and become overheated cannot perform at their best, and may collapse. Firefighters, workers and athletes are all at increased risk of heat stress in hot environments because as they use their muscles they have less blood available to flow to the skin to release body heat. Using firefighters as an example, Denise Smith, PhD, of the Illinois Fire Service Institute describes it this way:

“To appreciate how the cardiovascular system responds to firefighting (and the related hyperthermia and dehydration), it’s useful to consider an analogy between a fire engine and the human cardiovascular system. An engine relies on water flow from a hydrant and pumps water through a hose that delivers water to extinguish a fire. The larger the fire, the greater the demand for water delivery. Similarly, in the cardiovascular system, the heart receives blood from the veins and then pumps it through the arteries to deliver blood to the body’s organs to sustain life.

Like a fire engine, when the cardiovascular system encounters heavy demands such as muscular work, heat stress or combination of both, the vital organs require more blood, in fact, every time when the demand is the greatest, or when a large amount of blood flow is needed since the firefighter is performing heavy muscular work and is under heat stress conditions, the heart’s pumping capacity is impaired.” (Denise L. Smith, PhD, “Hot Under the Turnout,” Fire Chief, August 2001.)

Avoiding heat stress

The first days in a hot environment are usually the hardest on workers, and this is when heat-related injuries most often occur. Workers must be gradually acclimated to working in hot environments, and must be reacclimated after an absence (see Appendix C).

Keeping well hydrated is a critical factor in preventing injury. The loss of 1-2% of body weight from sweat - equivalent to 1.5 pounds in a 150-pound person – is enough to trigger heat stress. Dehydration is easier to prevent than treat. Much of our dehydration is considered voluntary. During periods of intense activity, workers may become so focused on their task that they forget to hydrate. All the body’s fluids – sweat, blood, semen, saliva, body tissue, and digestive juices – make up one large body fluid pool. Loss of fluid from one source affects the level of the body’s other fluids. Unless replaced by hydration, profuse sweating will result in decreased blood volume just as bleeding or vomiting will. Thirst should not be relied upon as a measure of a workers’ hydration level – by the time thirst hits, the worker is already dehydrated.

Water is best absorbed when it is ingested in small but frequent amounts. The average person needs eight to ten glasses of water a day. Active people should increase their intake to two to four ounces during every 15 minutes of heavy activity. Before entering a hot environment, firefighters and other workers should drink copious amounts of water, and then drink more when leaving.

People can train themselves to drink more and minimize stomach discomfort experienced with excessive fluids. A drink’s temperature and flavor can double the amount consumed. It should be cool and should taste good. The mouth of the bottle should be large enough to gulp, not squirt.

The best indicator of proper fluid levels is urine output and color. Dark urine or lack of urination means the body is low on water and is trying to conserve its supply by hoarding fluid. This makes urine more concentrated and thereby darker.

The best way to prevent heat stress is to stay out of the heat. But if this isn’t possible, workers in all occupations can best protect themselves against heat injuries by understanding how vulnerable the human body is to heat, by recognizing the symptoms of heat stress, by keeping well hydrated, and by learning and practicing safe work habits.

Emergency Film Group has produced Heat: Working in Hot Environments which helps to train emergency personnel to work safely in protective clothing and under hot conditions.