No fire officer worth his or her salt would allow an apparatus to be driven to the point where the engine became so hot that it seized. Still, a blown engine can be replaced or repaired. Humans, too, will overheat, and repairing or replacing them is a very bad option.

Heat stress is a combination of environmental conditions, metabolic rate from activities and clothing worn that increases a body's core temperature. The human body tries to balance heat gain and heat loss, but when this balance is compromised, the body can't function at its optimal level.

Internal heat generated through body activity is the major source of heat gain. Only about 25% of this energy is translated to mechanical work; the remainder is released as heat in the contracting muscles. The harder the human body works during a particular activity, the more internal heat is generated.

Evaporative cooling — the evaporation of sweat from the skin's surface — is the major avenue for heat loss. When heat balance is compromised, the body will try to direct heat away from the core by increasing blood flow to the skin. Water secreted at the skin surface 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 evaporation is quite high, small amounts of sweat remove relatively large amounts of heat.

These fluids must be replaced as soon as possible to maintain normal body function. But by the time people experience thirst, they probably are dehydrated and potential functional capacity already will be reduced significantly.

The rate at which people sweat is determined by three main factors: acclimation, aerobic fitness and genetics.

Acclimation is a physiological adaptation that the human body makes with repeated exposures to heat stress during exercise. It increases the 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 of the repeated exposures during hot fire training. Regular and sustained aerobic exercise can elicit a similar response.

Firefighters who maintain an adequate fitness level will have reduced cardiovascular strain and a lower core temperature for the same level of heat stress. Fit firefighters also tend to have less body fat, which means that they don't have to carry around extra (non-functional) weight. They require less energy to do the same job. Body fat is a very good insulator and will compromise the body's ability to lose heat.

But no matter how hydrated, acclimated or fit a person is, if the sweat can't evaporate, then thermo-regulation will be compromised. It is 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 “Heat Stress Signals, Recovery,” below.) Firefighters who notice these symptoms should notify the officer in charge immediately and take appropriate action, including instituting work/rest cycles, keeping cool and avoiding radiant heat, drinking small amounts of appropriate fluids, and using fans and other cooling devices such as water spray bottles and damp towels. Ice packs aren't recommended.

Although technology and training have reduced significantly the number of deaths and injuries from heat strain, firefighters continue to face the dangers. The physical effects of heat strain are well documented. However, heat strain also directly affects a firefighter's mental agility and his or her ability to make decisions. The mental symptoms are not as readily identifiable, but the consequences can be fatal. Researchers studying Air Force pilot error found that the decision-making process is hampered significantly by increased heat strain, with decreased mental performance, vigilance and eye/hand coordination.

Highly motivated heat-stressed subjects exhibit a higher error rate, a narrowed attention span with neglect of secondary tasks, and a diminished response to unusual events. The potential for personal injury increases, as does the risk to anyone under the individual's command.

Stress limits for workers exposed to adverse thermal conditions recently have been challenged. Rather than basing exposure limits on a physiological criterion like increase in body temperature, changes in behavioral performance efficiency are a more sensitive reflection of human response to heat.

Reducing the risk of heat strain also will reduce cardiac strain. Approximately 50 U.S. firefighters die each year from cardiac arrest. Technology that reduces heat strain could improve these mortality rates.

Research shows that a breathable moisture barrier in a firefighter's ensemble can reduce risk from heat stress during moderate to light duties — 80% of the normal workload. With current technology, it is almost impossible to remove the build-up of body heat inside the protective clothing while protecting the outside from the real extremes of fighting a fire in an enclosed space.

For most tasks, a moisture barrier will allow firefighters to work longer without a rest, lower the increase in body temperature and reduce the risk of errors from poorer mental performance. New research is aimed at creating up-to-date clothing technology that will offer these benefits in the most extreme temperatures.

Jeffery Petersen is an Australian freelance writer now living in the United States. He served as a firefighter for eight years.