Evaporative Heat Loss

I learned one of my first practical lesson in first aid and hypothermia many years ago. I was at junior officer training school and we were going on a winter exercise in the mountains. The third day went off as usual, with long ski marches and a few occasional attacks from an “enemy.” that was our instructors. Our long ski marches were conducted the traditional way, 50 minutes of march, and 10 minutes rest, continuing for hours the whole day. And occasionally at night.

Setting up our bivouacs for the night, four soldiers in our troop was sent out in guard posts to protect our base. Two and two in pairs. The rest of us was setting up camp, very heavy work in considerable snow. After a few hours, one of the soldiers at a guard post, came rushing to the camp, yelling that his fellow guard soldiers were unconscious at his post. I was in the nearest tent, running in heavy snow with my buddies to aid the unconscious soldier.

We found him cold at the post, breathing but barely conscious. Just mumbling, no words, just sound coming from his mouth. Being near twenty years all of us, we had not much experience at all. Still, we believed this was a matter of life and death. We gave it all, feeling our buddy would die if we didn´t. Hauling through heavy snow and into our tent. We stripped him of all his clothes, put him into one of our already warmed sleeping bags, and put our gas stove on max. Then we wake our officers. He was later evacuated by air ambulance, my future workplace several years later.

It become apparent that the soldier had slept at his post. His buddy let him sleep, believing he was a good buddy, taking all the responsibility for the guarding of our camp himself. But after a long day of hard ski march, the soldier’s body was full of sweat. Suddenly stopping the activity with sleeping at the guard post, made him severely hypothermic. Making matter worse, his wet clothing consisted of a cotton t-skirt as the layer closest to the skin.

Now many years later, I think back on this incident with a different view and more wisdom. First of all, we consistently use woolen underwear when doing outdoor activities. Or other types of garments that transport the evaporation away from the skin. We did back at that time, it is “common sense” among all Norwegians. Nevertheless, the hypothermic soldier had chosen a cotton t-shirt on that particular day. Not a wise decision, a lesson we all learned the hard way.

So why is a cotton t-shirt a bad idea during outdoor activities in winter?

Probably, the most influential factor is because it leads to vast amounts of evaporative heat loss. While woolen shirts transport the moist away from the skin, the cotton t-shirt does not. So the skin is wet, and the body must now use heat to evaporate the moisture. The thing is, evaporation leads to massive amounts of heat loss. I will try to give you an example.

In the human body, all is about balance. Or homeostasis as the medical term is. Regarding heat, this means that there is a balance between gaining heat and losing heat. If any of them are more influential, the core temperature will eventually rise or lower. So if the heat loss is large, the core temperature is lowered. In rest, the human body produces approximately 100watt of heat. If we start shivering, we can have as much as 300watt of heat loss. If we engage in intense activity, we can produce more than 1000watt. Weighing in on the other end of the scale, evaporative heat loss can lose as much as 2500watt. Although the most patients will not have a heat loss this significant, it is still substantial. And it is the main reason why evaporative heat loss must be reduced as much as possible.

This has implications for both how we dress during winter activities, but also for rescue operations. When rescuing a hypothermic victim, reducing the evaporative heat loss is very important. For many victims, the evaporative heat loss is the most significant. I will write more about this in the future.

If you are a health care professional or have a special interest in this subject, I have written this for the general public. There is much more to say and learn about this. Comments are appreciated.