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Ask a scientist
In this new feature, NRC scientists answer questions about biology, chemistry, physics, astronomy and other science topics.
Question: What is hypothermia and why does it happen?
Answer: Hypothermia happens when your deep body temperature drops by 2° Celsius. Body temperature varies from person to person, so my deep body temperature might be 36.4° right now and yours might be 37°. In that case, hypothermia for me is 34.4°, whereas for you it's 35°.
When your temperature starts dropping, the body tries to counteract this. One of the first things it does is start shivering to increase heat production. Your body will also start to constrict blood vessels going to the surface of the skin and redirect that warm blood to your core. This does two things. First, your skin now acts as an insulator because it's not suffused with warm blood anymore. Second, because the body is redirecting warm blood to the core, there's less chance of heat escaping from that blood into the external environment.
If your temperature continues to drop and you actually enter hypothermia (that drop of 2° C), shivering will increase to the point where muscle function becomes seriously impaired. You may find it hard to perform simple tasks, such as opening a door, because your hands are shaking so much. Brain function also decreases, so your mental acuity goes down. You may start to think silly thoughts or make irrational choices.
Once you drop to about 30° C, you can become unconscious. Between 28° and 27° you may experience an irregular heart rate, which can lead to cardiac arrest. Once deep body temperature reaches 26° to 24°, it's usually fatal.
When it can happen
Hypothermia can happen in any situation where the rate of heat transfer to the environment is greater than the amount of heat the body is producing. It all depends on your level of activity, the clothing you're wearing and the external environment.
For example, if you're sitting in the bleachers at a football game on a cold day and you're all huddled up in blankets, wearing a winter jacket and a big wool cap, you'll probably be fine. The players on the field are not wearing as much thermal protection as you, but they're so physically active that they generate more heat than they lose to the environment. But if they stop moving and cool down too much, they could become hypothermic.
One common cause of hypothermia is falling into cold water. This is the focus of my research, which looks at how cold water affects human performance. Water conducts heat 23 times faster than air, so if you're immersed in, say, 5° C water, you have a much greater chance of developing hypothermia than if you're exposed to air at the same temperature.
Did you know?
The urge to urinate intensifies in cold weather. When the body redirects warm blood from the extremities to the core, blood volume near the heart increases, triggering nerve receptors. These receptors cause the body to offload some of that increased pressure by increasing urine production.
What to do
If someone is shivering violently and their skin starts to look pale, they may have hypothermia. If they're wearing wet clothing, cut it off right away. Then put them in a sleeping bag or wrap them up tightly in blankets. It's important to re-warm the person gradually so that you don't prematurely open up constricted blood vessels and shock the body by letting all that cold blood back in. Make sure they have enough thermal insulation that any heat they generate will be trapped and not released to the environment. When the body is ready, it will reopen the constricted vessels and let the blood back in.
There comes a point ? around 30° ' when the body doesn't even shiver anymore. This means the person's ability to generate their own heat is significantly impaired. The best thing to do in this case is to get in the sleeping bag with them and re-warm the person with your own body heat.
How to prevent hypothermia
?A person may become apathetic when they get very cold, so you might have to take matters into your own hands.?
Jonathan Power, NRC
The most important thing is to act right away if you begin to feel very cold. Zip up your jacket; put on a hat and gloves. Get indoors and take a break to re-warm. If you start to shiver, you need to act immediately.
If you're doing something physically active outside, wear clothing that wicks sweat away from the body. Sweat is water and it conducts heat quickly. For example, if you're chopping wood in the dead of winter, then you stop to do something else, you'll have a layer of sweat over your body that is going to conduct heat. So wear something that will keep the sweat off your skin.
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