The cotton lobby must be very strong. First of all, I love cotton, I love the feel and the comfort of cotton clothing. However, cotton has a dark side that outdoor activity folks know all about, but the general public (including my mother) don't seem to be aware of and in fact have a hard time believing.
It's so insidious, cotton feels so warm and friendly when it's dry, but when it's wet it becomes this cold, clammy, evil body heat stealing thing So I should rephrase the statement: Wet Cotton Kills
It doesn't have to be that way. More modern "wicking" polyester fabrics (such as Coolmax, Polartec, ...) will still insultate even when wet. I have been completely soaked through from rain and still warm. This is not the bad leisure suit polyester of the 1970's.
My friend Bill had a bad run it with cotton that he describes (with permission):
.... I excused my cotton Levis and
non-waterproof boots by thinking "it's just a short work session, how
bad can it get?" No rain pants, no polyprop underlayers.
Most of the day, whether it was raining or not, we were standing in
3-inch deep water. When it was raining, my levis got saturated, my
underwear too, and of course my socks (all COTTON).
I got really cold - I'd say hypothermic. We started about 10:00 and
didn't finish until about 4 PM. By about 2 PM, my thinking was muddy
enough that I couldn't contribute many ideas, and was mostly following
directions. By 3:30, I was reduced to standing miserably in a hangar
out of the wind, stomping my feet.
I should note this was at sea level at a populated location (an airport - packing up airplane parts). He was not in the mountains, not in the wilderness. It was during a torential rainstorm, but it wasn't so much the rain as just having wet cotton fabric laying on his skin that was slowly lowering his body temperature.
Fortunately he's fine, but he's going to be buying polypropoline stuff now.
It has to also be said that wool insulates when wet. It's just heavy when wet (itchy too). I personally prefer SmartWool.