Is it true outdoors women should never wear cotton on an outdoor adventure? The answer is: yes, most of the time.
In the world of outdoor adventure, you often hear the expression "Cotton Kills". The thought is that because cotton keeps moisture from internal (sweating) or external (rain) sources right up against your skin, it sets you up for hypothermia. So instead of wearing cotton we're urged to wear synthetics, such as capilene, thermax etc.; or wool, especially some of the newer types of wool that are much more comfortable and less itchy than the old kinds. Synthetics will "wick" the mositure away from you skin and dry quickly. And wool continues to insulate even when its wet. Not so cotton. It completely loses its insulating qualities and takes forever to dry.
But for outdoors women there is an additional consideration: underwear. If we shouldn't ever wear cotton, does that mean we need to buy new synthetic underwear? Capilene bras and underwear can be expensive. But if you're truly in a situation where hypothermia is a real risk, obviously it is money well spent.
So here are the situations where you really need to consider the investment:
1. Hiking in the mountains. When you hike in the mountains, you are gaining in elevation and temperatures are always cooler the higher you go. In addition you will be sweating with the effort of going up and mountain weather is notably fickle - this is the perfect set up for hypothermia. Avoiding all cotton is worth it.
2. You are on the water kayaking or paddling in cool weather. In this situation you are sweating, winds often come up, and you are always at risk of dumping and getting totally wet.
3. You're winter camping. It's hard enough staying warm without having damp clothes next to you. But regardless of what material your underwear is made of, you'll probably sleep warmer if you take it off in your sleeping bag.
When can you get away with wearing at least cotton underwear? It's OK if temperatures are going to stay above 60 degrees (although you will find that cotton t-shirts stay really clammy in humid weather); if your hike is a six hours or less and you have warm clothes in your car; or if you are carrying a change of underwear in your backpack (although here the problem is that your cotton underwear won't dry so you only have a dry change of clothes for one night).
And one thing you can always do is just take your underwear off if you start to get chilly. There is nothing more uncomfortable than a cold, clammy bra!