When people think of outdoor survival situations under adverse conditions they tend to think of survival knives, maybe guns, and other iconic tools of the action-hero survivalist. But the fact is that one of the greatest dangers you will face in the wild will not be from wild animals or head hunters…cold will kill you faster than anything else.

Experienced outdoors men and women like to talk about the rule of 3’s: A person can go for 3 weeks without food, 3 days without water, and 3 hours without warmth.

There is a lot of truth in this old rule, and you need to prepare carefully if you are setting out on any expedition. It might just be a camping trip, it might be a white water rafting trip, or you may be going searching for fossil bones. In any case, it’s better to be prepared than to face a situation without the necessary survival equipment.

Thermal underwear is a fantastic invention which seriously can save your life. Many professional athletes and adventurers wear thermal underwear not just to stay comfortable, but also to avoid injury. If you’ve never worn thermal underwear before you may suspect that it is itchy or uncomfortable, but modern thermals are smooth and in fact barely noticeable once you have them on. There are three main types of thermal underwear that you can obtain:

The cheapest kind of thermal underwear is usually of the acrylic fiber type, which is a synthetic fiber. Acrylic fibers  are relatively inexpensive, and also quite hard wearing. The downside can be that synthetic fibers absorb body odor, and can be difficult to refresh.

Also fairly affordable, is cotton thermal underwear. You need to be careful with cotton underwear though, as cotton has a distinct disadvantage in the field. When it gets wet, it completely loses its ability to insulate you, and is in fact worse than useless. Cotton thermal underwear is only appropriate for fairly light duty applications, and should not be worn into serious survival situations.

By far the best type of thermal underwear is pure wool. It might surprise some people to learn this, but wool has been used by civilizations all around the world for countless thousands of years for a good reason – it has amazing natural insulating properties, and it is also naturally antibacterial. Even when wet, wool maintains the ability to insulate your body, and as such is the best choice when you are embarking on a quest into the unknown.