Temperature's Rising
Heat stress refers to the combination of factors that increase core body temperature: environmental conditions, clothing and metabolic rate. Our bodies try to achieve a balance between heat gain and heat loss, but when this balance is compromised the body is unable to function at its optimal level.
The major source of heat gain is the internal heat generated through activity the body undertakes. Of the energy expended during this activity, only about 25% is translated to mechanical work; the remainder is released as heat in the contracting muscle. The harder the human body works during a particular activity, the more it generates internal heat.
The major avenue for heat loss is evaporative cooling through sweat. When heat balance is compromised, the body will try to direct the heat away from the core by increasing blood flow to the skin. At the skin's surface, water secreted from sweat glands absorbs heat from the skin, changes from a liquid to a vapor and is carried off by the surrounding air. Because the heat of vaporization is quite high, small amounts of sweat remove relatively large amounts of heat.
The rate at which people sweat is determined not only by genetics, but by hydration, state of acclimation, and aerobic fitness.
Hydration is the simplest to explain: You can't sweat if your body doesn't have enough water. The fire service the world over knows that to maintain normal body function, fluid must be replaced as soon as possible. It's also well-known that by the time you experience thirst, you are probably already dehydrated. That's when potential functional capacity will be significantly reduced.
Acclimation is a physiological adaptation that the human body makes with repeated exposures to heat stress during exercise; it increases our rate of sweat production, shortens the time it takes for sweating response to start and conserves sodium. Some of the best-acclimated firefighters are breathing apparatus training officers because they're repeatedly exposed to heat during fire training.
Regular and sustained aerobic exercise can elicit a similar response. Firefighters who maintain an adequate level of fitness will have reduced cardiovascular strain and lower core temperature for the same level of heat stress. Fit firefighters also tend to have reduced levels of body fat, which means that they don't have to carry around extra non-functional weight. In other words, less energy will be required by a fit person to do the same job as a less-fit person. Body fat also is a very good insulator and will compromise the body's ability to lose heat.
Of course, it doesn't matter how hydrated, how acclimated or how fit a person is — if the sweat cannot evaporate, then thermo-regulation will be compromised. It's therefore essential that firefighters are aware of the signs and symptoms of heat stress so that it can be identified early and the appropriate measures taken. (See sidebar, at right.)
At the first sign of any of these symptoms, firefighters should notify the officer in charge, who will take appropriate action, which can include:
- Instituting work/rest cycles.
- Keeping cool and avoiding radiant heat.
- Drinking small amounts of appropriate fluids.
- Avoiding coffee, tea and alcoholic beverages.
- Using water spray bottles, damp towels and fans. Ice packs are not recommended.
Although technology and training have played a significant role in reducing the number of deaths and injuries from heat strain, it's a danger that firefighters continue to face.
The physical effects of heat strain have been well documented. However, recent research shows that it can have a further deleterious effect on a person's mental state. Heat strain has a direct effect on a firefighter's mental agility and ability to make decisions. The mental symptoms aren't as readily identifiable in the heat of a large fire, but the consequences can be fatal.
Research in this area has been led by the interest in reducing airplane pilot error. Researchers found that the decision-making process is significantly hampered by increased heat strain, with consequences such as decreased mental performance, vigilance and hand-eye coordination.
Highly motivated heat-stress subjects exhibited a higher error rate, a narrowed attention span with neglect of secondary tasks and a diminished response to unusual events. The potential for injury to the individual and to those under his or her command is therefore much higher.
Stress limits for workers exposed to adverse thermal conditions recently have been challenged. Rather than basing exposure limits on a physiological criterion such as an increase in body temperature, it's suggested that changes in behavioral performance efficiency are a more sensitive reflection of human response to heat.
Reducing the risk of heat strain also will reduce the incidence of cardiac strain. In the United States, approximately 50 firefighters die each year as a result of cardiac arrest. Improved technology to reduce the risk of heat strain could have a positive affect on these mortality rates.
Current technology is such that in the real extremes of temperature experienced when fighting a fire in an enclosed space, it's almost impossible to remove the buildup of body heat inside the protective clothing while still protecting thoroughly against outside elements.
General research and specific research undertaken by W.L. Gore and Assoc., manufacturer of Gore-Tex waterproof and breathable fabric, have shown that the inclusion of a breathable moisture barrier in a firefighter's ensemble can significantly reduce risk from heat stress for a firefighter carrying out moderate to light duties, about 80% of the normal workload.
For most of the work undertaken by firefighters, a moisture barrier will have a distinct effect, allowing them to work for longer periods without to rest, lowering the increase in body temperature and reducing the risk of errors through poorer mental performance. Research is currently aimed at creating new clothing technology that will offer these benefits in the most extreme temperatures.
Jeffery W. Petersen is an Australian freelance writer and a former eight-year veteran firefighter now living in Billings, Mont.
Symptoms: Painful muscle cramps, especially in abdominal or fatigued muscles.
Signs: Incapacitating pain in voluntary muscles.
First aid: Rest in cold environment, drink salted water (0.5% salt solution), massage muscles.
Symptoms: Blurred vision (gray-out); brief fainting.
Signs: Brief fainting or near-fainting.
First aid: Lie flat on back in cool environment, drink water.
Symptoms: No early symptoms, fatigue, weakness, dry mouth.
Signs: Loss of work capacity, increased response time.
First aid: Fluid and salt replacement.
Symptoms: No early symptoms, fatigue, weakness.
Signs: High pulse rate, profuse sweating, low blood pressure, insecure gait, pale face, body temperature normal to slightly increased.
First aid: Lay flat on back in cool environment, drink water, loosen or remove clothing.
Symptoms: Chills, restless, irritable.
Signs: Red face, euphoria, shivering, disorientation, erratic behavior, collapse, unconsciousness, convulsions, body temperature more than or equal to 40° Celsius.
First aid: Immediate, aggressive, effective cooling, transport to hospital.
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