Hot, sweaty mess


generally measure similar variables that can be used to indicate the onset of heat strain. They all measure electrocardiogram (ECG) and heart rate variability, respiration – breathing rate and volume, and some of them will even measure skin temperature, although in single location only. They also measure some other useful indicators that help to predict heat strain, like activity levels, from gyroscopes and accelerometers. All these sensors collect data that is then interpreted by algorithms which refer to various heat strain indexes to indicate whether the wearer is approaching “dangerous” rather than just uncomfortable levels of heat stress. “They don’t directly say ‘you are too


hot,’ but they can infer that from the algorithms. The only way to really know if you are reaching dangerous core body temperature levels is still to swallow a monitoring pill. These are getting smaller and cheaper now and some have tiny radio-frequency identification (RFID) tags in them so their data could be integrated into the RPM system too. That would give you the best indication of core temperature.” At risk of seeming squeamish, the


idea of swallowing a RFID tag does not excite me, falling into the ‘stuff about my body that I never needed to know’ category. Despite these qualms there is an advantage to having core body temperature in real time without anything being inserted in me where things shouldn’t be inserted. That data would accurately predict when you will be feeling hot and start to ramp up the cooling to a level that will have a significant impact. “There’s more to physiological


burden than simply how hot you are though. What we’ve learned from various studies on firefighters is the danger from cardiac and stroke events. Heat contributes to this, in part by causing high levels of sweating, making the blood thicker and stickier and of course the PPE worn and the work itself demand high levels of physical exertion. Dehydration coupled with an elevated heart rate is, quite literally, a killer combination. In fact, a relatively recent (2015) CBRN study in Australia looking into physiological tolerance


times in CBRN PPE terminated almost 80% of the trials when the subjects’ heart rate reached dangerous levels, but only around 6% had to be stopped due to core temperatures exceeding 39°C. Cooling down fully, re-hydrating and resting until your heat rate has returned to normal levels after activity in CBRN PPE is critical and RPM systems can predict the need for rest periods and tell you when it’s safe to get back to work as they monitor your recovery rate. Due to the unique structure of blood vessels in your hands and feet, dunking them into a bucket of water is a really effective way to quickly cool down. It doesn’t need to be (and in fact shouldn’t be) very cold water, just cooler than your skin to work effectively.” “We know that heat strain in CBRN


PPE is caused by a combination of factors; the environmental temperature, the PPE worn and levels of physical effort. When you can’t change the environment or the PPE, you can at least manage work and rest periods and that’s where RPM and cooling systems can be very useful for extending safe operating times and wearer comfort levels.” As ever, there is a trade off when it


comes to how much cooling is possible, for how long, and how much burden you want to tolerate. Phase change packs need to be pre-cooled, they add bulk, stiffness and weight, but can be effective for short periods or pre- cooling. Air cooling systems tend not to be very effective, especially in hot environments. If you can get airflow and movement to help evaporation off your skin, or off the layers close to your skin though, you can achieve some evaporative cooling. Usually evaporative cooling is the most efficient way to cool the body, but the problem comes when you’re in a hot environment and/or you wear CBRN kit. Unless you are wearing something permeable (and even then you may have impermeable layers of armour or equipment vests covering the torso) the moisture can’t escape. Heat is only lost at the point of evaporation, so when moisture is trapped inside your ensemble, so is the heat. Your skin might feel dryer if the sweat is wicked away, but it hasn’t actually cooled


unless it has evaporated off. If it condenses inside the suit, it releases the heat back into the suit’s microclimate. There’s a limit to what you can achieve with evaporative cooling in CBRN PPE. Moreover, when the external temperature is over 34°C, the body’s usual thermoregulation mechanisms don’t work and instead, the body gains heat from the external environment. In these situations, liquid cooling systems are effective, using conduction and convection to duct significant amounts of heat away from the body, but of course, they require a lot of power. “If you can keep core temperature in


a normal range, not only will you lower the health risks of working in CBRN PPE, but you can work harder for longer too. When your body isn’t diverting blood flow to the skin for cooling, it’s delivering oxygen to your muscles (including the heart) so they can work more efficiently. When you aren’t sweating, you aren’t losing the electrolytes that your muscles and nervous system need to function properly and you also lessen the risk of blood clotting. You might still feel bloody hot, but the difference is that it isn’t dangerously hot.” One of the soft issues with handing


someone’s physiological monitoring information is what you do with it. Providing vital health information means that the commander is potentially more aware than you are of what your body can, or cannot, achieve. At one end of the scale this is persuading an individual that they are not too hot to go down that corridor, the opposite end is a court case for manslaughter after it turns out that the individual was too hot after all. This requires the commander to understand the impact of heat stress on the body at a far higher level than they are likely to possess and to appreciate that the mission might have to come second. Cath Rogan agreed it was difficult,


and there had been reluctance from some user communities to embrace the technology because of these issues. “Fortunately, the RPM systems can be calibrated to individual users’ specific physiology, so the threshold indicators do take into account that one user may


CBRNe Convergence, Orlando, USA, 6-8 November 2018 www.cbrneworld.com/convergence2018 56 CBRNe WORLD February 2018 www.cbrneworld.com


CBRNeWORLD


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