Turnout gear, also known as thermal protective clothing (TPC), is an encapsulating garment that is typically worn with respiratory protection. These garments protect the firefighter from external heat and flame but the tradeoff when donning these garments is impaired ability to dissipate internal heat created from the exertion during fireground and rescue operations. These garments also impair movement and reduce the firefighter’s maximal workload.
Chemical protective garments, such as Tyvek, are lighter than TPC but the formation of a complete barrier to evaporation makes it nearly as stressful. Regardless of the risks associated with TPC, these garments must be worn when working in hazardous environments and the characteristics of these garments is unlikely to change in the near future. Therefore, it is important to understand the effects of TPC on the firefighter and plan ahead to mitigate the heat stress.
The body is not a single temperature from the skin to the core. The brain manipulates the body systems to keep the core temperature higher than the skin temperature. By maintaining this gradient, we can transfer warm blood from the core to the skin when we are too hot and pull blood away from the skin if we become too cold. The firefighter is at risk of injury if core temperature rises faster than heat can be dissipated through the skin. There are five ways a human can gain or lose heat in order to maintain body temperature in an acceptable range. The most important method for shedding excess heat to the environment is to increase blood flow through the skin, produce sweat, and allow the sweat to evaporate. The process is normally very efficient unless the environment is very humid or the skin is covered by garments, both of which inhibit evaporation. The firefighter suffers both of these issues when wearing TPC. The sweat produced as the firefighter’s temperature rises cannot efficiently evaporate through the TPC creating a wet environment between the TPC and the firefighter’s skin. Additional heat stress is suffered when the radiant heat from the fire or the sun warms the firefighter externally.
Sweating can further aggravate heat stress by reducing the amount of blood available to carry oxygen to the working muscles. Sweat is produced from the liquid component of blood and heavy sweating causes a progressive dehydration. This can be quite rapid during fire suppression. In our experiments, a single 20-min. training fire can result in a firefighter losing as much as 1-kg of body mass from sweating. Longer bouts of lighter intensity work in TPC can result in nearly 2-kg of lost body water from sweating. Dehydration forces the body to work harder to accomplish any given task. The heart must beat more rapidly to move the reduced volume of blood to the working muscles. As dehydration becomes more severe, the amount of blood that can be diverted to the skin is reduced and heat stress becomes worse.
The sum of these physiologic challenges is exertional heat stress (EHS). High heart rate, high core temperature, rapidly rising skin temperature, and rapid breathing characterize EHS. This condition can occur in many settings of sport and occupation but, for the reasons discussed above, the firefighter can do the least to avoid or slow the onset of EHS. Figure 1 most clearly demonstrates EHS and the effect of protective clothing on physiology. In this graph of heart rate during exertion, a young healthy male carried a 4.5-kg tool and walked on a treadmill at a comfortable pace and mild incline. After 10 minutes, he walked more slowly on a level plane for five minutes to allow for partial recovery. This was repeated three times. The red line shows the subject’s heart rate response when wearing short pants and a short sleeve athletic shirt. Notice that the heart rate response is very consistent through exercise. In comparison, the blue line shows the heart rate response when wearing a Tyvek coverall and filtering facepiece respirator. Each subsequent bout of work results in a higher heart rate and incomplete recovery. By the end of the third bout, his heart rate was nearly maximal even though he was still at a walking pace. This is the picture of exertional heat stress. The subject was sweating heavily under the garment. The sweat could not evaporate so the subject has less circulating blood volume causing the heart to beat more rapidly and the subject’s core and skin to rapidly rise. Firefighters subjected to this type of stress without adequate rest and recovery intervals will become too fatigued to continue working or suffer a heat illness.
Heat stress during fireground and rescue operations is unavoidable. Devices that can be worn under TPC to cool the firefighter are either not practical or not effective. Heat stress must be mitigated at regular intervals by rehydrating and by providing a cooling interval. There are a few simple rules that can be applied to enhance recovery after exertional heat stress. The first and most fundamental is to remove as much TPC as possible. These garments contributed to the exertional heat stress. Wearing them during the recovery interval is counterproductive. At a minimum, the firefighter should remove helmet, hood, coat and gloves. There is an additional benefit to pushing the TPC pants down around the boots when the firefighter is sitting. Temperature will be high at the beginning of the rest interval so removing garments and rolling up long sleeves will allow the sweat to evaporate. Remember that evaporation is impeded by humidity so you may have to move the recovery area indoors or into an air-conditioned vehicle when the ambient humidity is high.
Allowing the firefighter to passively cool is effective when TPC is removed and the recovery area is placed in a cool, non-humid environment. We conducted a study comparing five cooling devices to passive cooling in a room temperature setting and found that all modes, including passive cooling, worked equally well (Figure 2). It is worth noting that temperature does not return to baseline after 20-30 minutes of rest. Firefighters who perform another bout of work after the rest interval are starting at higher heart rates and temperatures than normal and, after the recovery interval, they most likely will not work as long or at the same intensity as the previous bout of work.
Passive cooling, however, will not be as effective in hot or humid conditions. In these situations, a fire department will have to find a rest area that is indoors or in a vehicle or choose to invest in active cooling devices. One of the simplest devices available are 20 L buckets filled with cool water that the firefighter uses to immerse his hands and forearms. This form of conductive cooling removes heat from the blood in the superficial veins before it returns to the central circulation. Over 15-20 minutes, this can be very effective at cooling the body (Figure 3). Research from our lab and others has shown that other devices, such as cold towels and fans, are largely ineffective when the environment is hot and humid.
Rehydration is an important part of recovering from exertional heat stress. As shown above, the fluid lost from sweating while working in TPC is substantial. Most firefighters will not drink enough fluid during the rest interval to fully rehydrate. In fact, it may not be possible for many individuals to consume that much fluid in a short period of time without suffering gastric distress. The individuals must be counseled to continue drinking after the incident concludes in order to be effective during the remainder of the shift. The type of fluid that should be consumed is a hotly debated topic. Most advocate using plain water for rehydration although a very recent study reported that certain beverages such as milk and electrolyte solutions remain in the system longer than water when consumed after exercise. Beverages with high sugar content, such as soft drinks, should not be used to rehydrate after heat stress, as there is the potential for mild kidney damage.
Lastly, it is important to understand what can be accomplished in the recovery interval. Firefighters in our studies often report feeling fully recovered 10-15 minutes into the recovery period. This is probably due to the rapid skin cooling that occurs when TPC is removed. In fact, good rehydration and cooling practices only partially corrects the physiologic strain caused by exertional heat stress. It cannot reset the firefighters physiology back to baseline. The residual heat and dehydration after the recovery period will not allow the firefighter to work at the same intensity or duration as the previous bout (Figure 3). Prolonged incidents will require more manpower to minimize the chance of a firefighter becoming overly fatigued and suffering an injury.
In summary, firefighters at every level must understand the physiologic strain caused by TPC and working on the fireground. The most important lesson to learn is that TPC should be removed every time it is safe to do so. This will prevent further heat stress and may correct some of the heat stress that has already occurred. Firefighters should work to maintain normal hydration throughout the shift. Do not overhydrate, but regularly drink small amounts of fluid throughout the shift and somewhat larger amounts when you are sweating. It is a good practice to drink a liter of fluid in the hour following an incident to be prepared for the rest of your shift. Lastly, getting hot is part of firefighting but it is not healthy to have high body temperatures for long durations. Take every opportunity to cool off and be prepared to deploy an active cooling strategy when the weather conditions will not permit efficient passive cooling.
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