The current firefighting protective ensemble is heavy, thick, multilayered, and it exacerbates the challenge of thermoregulation due to its limited permeability and insulative properties. A person's capacity for balancing the heat stress by evaporative cooling is truly remarkable but for firefighters, the standard clothing will limit the rate of evaporative cooling. The protective barrier of the gear also prevents the body from properly cooling itself. The generation of internal body heat and the inability to dissipate heat are further compounded by the weight of the gear (bunker pants and jacket, helmet, and self-contained breathing apparatus), which approaches 53 pounds. Add to this that the firefighter must be able to transport various pieces of equipment that can weigh over 77 pounds while still in full gear. In light of the inherent trade-off between personal protection and the well documented cardiovascular and thermoregulatory strain associated with firefighting activities, there are ways of keeping firefighters cardiovascular strain below critical levels during firefighting activities. Due to the nature of a firefighter's work and working conditions, it is inevitable that firefighters will increase their core body temperatures. The problem is how to prevent this from becoming dangerous to them.
Physiological Strain Of Firefighting
The two physiological systems most frequently monitored during firefighters research is: heart rate and body core temperature. It is well known that heat stress represents an additional load on the cardiovascular system. This is evidenced by an elevated heart rate at the same work rate in a hot environment versus a room temperature environment. Firefighting imposes a considerable amount of both physiological and psychological strain. For example, one research study reported that when firefighters wearing standard turnout gear performed two firefighting tasks (advancing fire hose and chopping wood) while inside the training structure. At the completion of the test, including both tasks, the average heart rate was 182.3 beats per minute and temperature 104.1 degrees F, suggesting that the firefighting tasks used in this study impose considerable physiological strain on firefighters (1). Additionally, the perceived workload during the fire drill was near maximum suggesting intense psychological stress as well.
In another study looking at the cardiovascular stress placed on firefighters in response to a live fire training exercise which involved dragging an 178-pound dummy for 15 minutes along a corridor and down a flight of stairs resulted in a near maximum heart rate during the 15 minute drill (2). So what causes a firefighter's heart rate to skyrocket during intense heat? A basic principle of cardiovascular physiology is measured by cardiac output: the total volume of blood pumped by the ventricle per minute, or simply the product of heart rate (HR) and stroke volume (the amount of blood ejected from the ventricle per beat). Studies have observed that when firefighters are exposed to intense heat, they become dehydrated which results in a reduction in plasma volume (i.e. the watery portion of the blood) which results in a reduction in stroke volume and an increase in HR. Stroke volume has been shown to decrease by 35% following three seven-minute bouts of strenuous firefighting tasks in the heat (3). It is suspected that when plasma volume decreases during intense sweating this "thickens" the blood and may be a contributor to the heart-related increase. Plasma volume is affected by hydration status: the more dehydrated a person is, the less plasma volume they have and a higher heart rate is observed. Firefighters should learn to consume water liberally throughout the day because an fire alarm can be activated at any time.
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.
Most firefighters are on duty for 24 hours, and then they are off for 48 hours and during this time they live at the fire station. Since there is never a "planned emergency," they are always anticipating the alarm. Once the alarm sounds there is an automatic adrenaline "rush" and an increase in their heart rate, before they even get to the fire. Combine the emotional stress of this with the health stresses of being overweight and having low cardiovascular fitness, and the physical stress of fighting the fire, overweight firefighters are more at risk for a heart attack. Interestingly, almost 50% of on-duty firefighter fatalities are not from direct fire related injuries but from heart attack and stroke. A firefighters's aerobic capacity is probably the strongest single determinant of an individual's ability to carry out sustained physical work under hot conditions.
As noted above, early findings of group differences in heat tolerance which were attributed to age are now viewed as manifestations of aerobic capacity and heat acclimatization. In a study conducted by the NFPA from 1977 through 1995, about one-half of all line-of-duty deaths of firefighters were not the result of fires, but heart attacks. In almost all of the cases, the heart attacks suffered by firefighters are directly linked to the exertional demands of the firefighter's job. Nationally, figures show three-fourths of firefighters over the age of 45 who die in the line of duty die from a heart attack. And of the firefighters who have died from a heart attack while in the line of duty, 2 of every 5 had documented heart conditions. Firefighters place considerable strain on the cardiovascular system. Since a fire alarm can come at any time, it's important for firefighters to pre-hydrate the body by drinking six to eight glasses of water per day. The risk of cardiovascular strain can be minimized by being well hydrated, having a high aerobic capacity, and not having excess bodyfat.
Sources:
- Smith DL, Petruzzello SJ, Kramer JM, Misner JE. Physiological, psychophysical, and psychological responses of firefighters to firefighting training drills. Aviat Space Environ Med. 1996 67(11):1063-8.
- Eglin CM, Tipton MJ. Can firefighter instructors perform a simulated rescue after a live fire training exercise? Eur J Appl Physiol. 2005 Oct;95(4):327-34. Epub 2005 Oct 27.
- Smith DL, Manning TS, Petruzzello SJ. Effect of strenuous live-fire drills on cardiovascular and psychological responses of recruit firefighters.Ergonomics. 2001 Feb 20;44(3):244-54.
- Eglin CM, Coles S, Tipton MJ. Physiological responses of fire-fighter instructors during training exercises.Ergonomics. 2004 Apr 15;47(5):483-94.
- Faff J, Tutak T. Physiological responses to working with fire fighting equipment in the heat in relation to subjective fatigue. Ergonomics. 1989 Jun;32(6):629-38.
- Womack, J, Green, J.S., Crouse, S.F. Cardiovascular Risk Markers in Firefighters: A Longitudinal Study. Cardiovascular Reviews & Reports, 21;10:544-548 (October, 2000)
