If you haven't heard the outdoor adage "cotton kills," pay attention and you may just save your life. Sure, cotton is fine to wear while shopping for groceries and looking "outdoorsy," but wear it in the great outdoors and it may kill you.

Every outdoor organization from the Boy Scouts to The Mountaineers warns vehemently against wearing cotton while hiking and backpacking. Despite multiple warnings from these experts, people perish each year from inadequate preparedness whilst in pursuit of outdoor adventure.

A guide friend of mine is so anti-cotton that he inspects each garment of everyone he is taking into the backcountry. He's been known to say, "take in an ounce of cotton, and pack out a cold body." A bit dramatic, yes—but is it well founded?

While teaching backpacking classes, my favorite illustration of the dangers of wearing cotton is to soak two pairs of pants—jeans and supplex hiking trousers—and let them sit out overnight. In the morning, the class gasps as the jeans creak and moan as I bent their frozen form in half. Meanwhile, I just shook the ice crystals off the hiking pants.

So which would you rather wear?