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There's a common saying in the hiking community that "cotton kills." The reason why has to do with moisture management in cold and cool conditions. In fact, there is absolutely nothing wrong with wearing cotton when you can stay dry and warm. The problem is, it is much harder to stay dry and warm when hiking than most people think.
Problems with cotton occur when the cotton gets wet. Cotton does not wick moisture and can become abrasive when wet. When this happens you must watch out for blisters and packsores in both warm and cold weather. Because cotton holds so much moisture, it can hold that moisture against your body and sap body heat from you. This can quickly lead to hypothermia, even in temperatures well above freezing. Cotton also gets heavy when wet. If you wear cotton while hiking, you generally need to carry more "emergency" clothing in case your cotton does get wet (sweat, rain, or falling in while fording a river will all get you wet).
Silk also loses it's insulative properties when it gets wet and does not wick like modern hydrophobic fabrics.
Wool insulates relatively well when wet however while some weaves do shed water for a period of time, wool eventually absorbs a great deal more moisture than comparably weighted synthetic garments and becomes very heavy.
All of these natural fiber fabrics take much longer to dry once wet than comparably insulating (or thin) synthetic fabrics.
The feel of synthetic fabrics has changed quite a bit from the first dark blue polypropelene long underwear I bought. Different brands have different feels. Try a few and see what feels good to you. I happen to like Capilene from Patagonia (both silkweight and other weights) and try to get it on sale (usually at the end of the season when they are trying to get rid of colors that didn't sell.)
So, if you choose to wear natural fibers, think ahead and be prepared for situations when you get an unintentional soaking. Remember hypothermia can set in at temperatures well above freezing.
Last updated, January 4, 2016.
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