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.