At body temperatures substantially higher than the optimal level (97.5 degrees F to 99.5 degrees F), both physical and mental performance may deteriorate due to the complicated interplay between physiological processes. Heatstroke is extreme hyperthermia, typically above 100.4 degrees F, associated with a systemic inflammatory response, which leads to end-organ damage with universal involvement of the central nervous system. Body temperatures above 104 degrees F are life-threatening. At 106 degrees F, brain death begins, and at 113 degrees F death is nearly certain. In a recent study, the physiological responses of fire instructors to a live-fire training exercise were found to vary considerably due to the differences in heat tolerance and aerobic fitness level of the firefighters. At the end of the live fire training exercise, the body temperature of the instructors averaged 38.5 °C and in eight out of the 26 exercises it exceeded 102.2 F (4). The study suggests that even during training exercises, body core temperatures are approaching critical levels and more than likely during real life encounters body core temperatures will rise even higher.
It should be noted that brain function is particularly vulnerable to heat stress. For example, in a series of experiments, scientists exposed men to two conditions (i.e. room temperature and heat stress) and evaluated random movement generation. Random movement generation deteriorated significantly from pre- to post-treatment in the heat condition only. The deterioration in cognitive performance was directly related to the magnitude of dehydration and rehydration had some effects on returning cognition to normal. Intense heat strain may cause firefighters to suffer a loss of concentration, which then leads to accident and injury. Thus, preventing heat stress in order to make quick decisions is of critical importance during firefighting activities. Tolerance to elevated deep body temperature is prolonged if the brain is kept cool. The most serious consequence of exposure to intense heat is heat stroke, which may be fatal. It is caused by a sudden collapse of temperature regulation, leading to a marked rise in body heat. The rectal temperature may be 105.8 degrees F or higher. If the normal signs of heat stress, such as thirst, tiredness and visual disturbances are not heeded, the result can be a series of disabling complications such as heat cramps, heat exhaustion, and heat stroke. A large number of injuries may be caused by mental and physical decrements as a consequence of poor hydration; however, the primary cause (dehydration) is misdiagnosed.
Aerobic Fitness: A Key Component For firefighters Health
Firefighters must go from rest to a sudden, very physical, high-stress situation that requires them to be able to perform duties as needed. Firefighters must be able to lift and hold, for extended periods of time, various pieces of equipment that may weigh in excess of 77 pounds. They are also required to ascend ladders in full gear, while carrying additional pieces of equipment of various weights. Therefore, strength and muscular endurance are of utmost importance to the firefighter. Recent work from muscle physiology laboratories have reported that maximal muscle power is significantly reduced following exercise-induced hyperthermia. The heavy physical labor that firefighters must perform can result in heart rates of 88±6% of the predicted maximum. It should be of interest to firefighters that work heat tolerance is determined by aerobic capacity. This is supported by a study where firefighters exercise tolerance time in the heat was not related to firefighters age but to aerobic fitness levels (5).
A 2002 study found that most firefighters tend to have a decline in aerobic capacity from when they start at the station. In that study, a group of firefighter's aerobic fitness level and bodyfat was tracked over a 6-year period. At the end of the study, the firefighters's aerobic capacity was found to be below age predicted averages, and body fat percentage was found to be significantly above age-predicted averages (6). Regular aerobic exercise will reduce cardiovascular strain during firefighting activities. Additionally, regular aerobic training will increase the watery portion of the blood (i.e. plasma volume) by 20- to 25-percent. Additionally, the amount of blood ejected per beat (i.e. stroke volume) of a physically fit individual can be 50-percent higher than an unfit, untrained individual.
Excess Bodyfat: An Additional Risk Factor For Firefighters
Generally, the chronic physical risks that firefighters face include being overweight or obese, or possibly having cardiovascular disease or osteoarthritis in the knee. Excess body fat is a liability when in a hot environment because the specific heat of fat is greater than muscles. Furthermore the insulatory property of fat retards the conduction of heat to the periphery. Therefore a firefighter with more fat mass is at a thermoregulatory disadvantage as he will have a faster rise in body core temperature during heat stress.