When you're doing strenuous work in a hot environment, it raises your body's core, which triggers profuse sweating as the body attempts to lower its temperature through evaporative cooling.
However, the encapsulating properties of your structural firefighting protective ensemble creates a warm, moist, stagnant microclimate around the skin, severely limiting the evaporation of sweat, and hence, the effect of evaporative cooling. Studies have shown that profuse sweating can decrease plasma volume, placing additional strain on the cardiovascular system and further impairing thermoregulation.
Historically, much emphasis has been placed on cooling firefighters when they report to on-site firefighter rehabilitation (fire rehab). But what if new technology can provide a cooling effect to firefighters while they're actively engaged in emergency operations or live training exercises?
For humans, 97.7-99.5 degrees F (36.5-37.5 degrees C) is a typically reported range for normal body temperature. Hyperthermia occurs when the body produces or absorbs more heat than it can dissipate and is usually caused by heat stress – prolonged exposure to elevated temperatures.
The heat-regulating mechanisms of the body eventually become overwhelmed and unable to deal effectively with the heat, causing the body temperature to climb uncontrollably.
Hyperthermia at or above about 104 degrees F (40 degrees C) is a life-threatening medical emergency that requires immediate treatment. The most common signs and symptoms of hyperthermia are listed in Table 1.
Eighteen minutes of simulated firefighting activity causes significant physiological disruption including an increased heart rate (75 bpm) and an increased core temperature (1.2 degrees F).
The simulated firefighting activities caused a significant increase in platelet numbers and a significant increase of platelet activation, resulting in faster clot formation.
Firefighting activity resulted in an increased level of coagulation (increased factor VIII activity and PF1.2) and significant changes in clot breakdown (fibrinolysis).
Simply put, heat stress causes physiological changes in the body, the most profound of which is a sudden cardiac event (SCE), such as a heart attack or stroke. In 2016, SCEs caused 39 percent of firefighter LODDs; over the past 10 years, SCEs were responsible for 42 percent of firefighter LODDs.
Enter phase change material (PCM). When a PCM is in its solid phase, it will absorb heat as the external temperature rises. The temperature of the PCM will mirror the external temperature until the PCM's melt point is reached. When the external temperature reaches the melt point of the PCM, the PCM will begin to melt (i.e., "phase change"). During the phase change process, the PCM will absorb large amounts of heat with almost no change in temperature.
If you're thinking at this point that ice is a PCM, you're right. But keep reading to learn why ice is not the best PCM).
This is a huge development for actively cooling firefighters because, during its phase change, the PCM is providing a cooling effect. Cooling vests that incorporate PCM cooling technology produce a significant cooling effect as the PCM absorbs body heat during its transformation from a solid state to a liquid state. These elements are not affected by humidity and react only to direct heat (e.g., direct contact with a firefighter's hot skin) and have far less bulk than vests using ice packs. Those elements will continue to absorb heat away from a firefighter's skin until the PCM reaches its liquid state.
The reverse is true, as well. Think of a firefighter getting heated up in their PPE while fighting fire inside a structure, and then going outside into a wintery evening. As the firefighter's skin temperature drops, the PCM elements, now in their liquid phase, can release the heat they absorbed while the firefighter was actively engaged and provide a warming effect.