"Cotton Kills" only in cold weather. Cotton clothing is particularly dangerous in wet conditions at temperatures between freezing and 50 F. We have a full sized plastic skeleton with a sign hanging from its neck that says, "Cotton Kills" displayed in our ski patrol's cabin. It is used to remind visitors that jeans may not be the best ski wear for backcountry skiing.
There is some debate about the value of cotton in hot weather, but from what I could tell from some cursory research it works very well in hot weather and low humidity. My wife and I have toured on 3 different tours in temperatures of 109 F. On our first encounter with those hot temperatures I had a synthetic T, but picked up a cotton T about half way through the hot zone. I did this based on what I had heard about cotton in hot weather. It might have been the placebo effect, but the cotton seemed more comfortable.
Do a search and you can find a lot of contradictory information about the pros and cons of cotton in hot weather.
Having said that, I only packed synthetic T's on the other 2 occasions and they did a good job in similar conditions. I believe color is also an important factor; white is a good warm weather color. My warm weather touring jerseys are white which seems to help.
On the other hand, my wife wears a light weight, almost a mesh, wicking long-sleeve T during hot weather. Note the "bloomers beneath the legs of her shorts. She also bought a 2X cotton T shirt and cut off the sleeves. She then put them on like leg warmers, tucking them under the legs of her shorts. This gave her upper legs good sun protection. She cut the rest of the T open and used it to sleep under when it did not cool down in the evenings. She has carried these on every summer tour since that trip.
