The Cotton Board has these beautiful commercials for their product. The color is great, the people are gorgeous and the music is cool. The only words are: "The touch...the feel...of cotton. The fabric of your life." Maybe that's true in ordinary circumstances, but not for anything where you might sweat at all. Yesterday, I (accidentally) took a 6-mile trek to see a waterfall. I didn't know that I would be trekking, so I was wearing a lot of cotton. It did not work!
Temp: Upper 70s, sunny, but in shady forest
Precip.,Humidity: No rain, moderately humid
Distance: 6 miles
Did it work: No. Everything got sweaty. Because cotton takes longer to dry, the wet fabric just sat against my skin, feeling cold when I stopped moving. The running on the way back was to warm back up after a pause to admire a waterfall. When the shorts got sweaty, they stretched out and were two sizes too big by the time I got home. The sandals held up surprisingly well, even when running.
We didn't know the walk to the waterfall was going to be so long, so it would have been hard to plan better. But, if you know you're going to be running, biking or walking for exercise, don't wear cotton if you can help it. You will end up a soggy, saggy mess with chaffing and possibly a cold due to poor body temperature regulation. Cotton may be the fabric of a sedentary life, but when it comes to being active, cotton is rotten.