Why don’t we run in cotton T-shirts? Apparently Pam Smith did when she won the 2013 Western States 100 Endurance Run. So why don’t I?
We can run in cotton, and at times in the past we did, but lets take a look at the properties of cotton and see why most running shirts are now made out of synthetic fibers, mainly polyester.
Cotton fiber properties
Cotton is a short staple natural fiber. “Staple” is the term used when the length of the fibers that make up the yarn are relatively short. Staple length fibers are measured in inches vs filament length that can be kilometers. A cotton boll produces fibers that are only a few inches long. They are spun together to make the yarn that is used in T-shirts. Cotton is naturally absorbent and breathable, which feels great against the skin when dry. However, cotton loves to absorb and hang onto that water. It stays wet. Therefore, when we sweat it holds onto that sweaty funk. It can also mold easily. So simply, we don’t run in cotton t-shirts because they become damp stinky sponges which can lead to chafing.
Using cotton to cool us off
However, if you are looking for something to help cool you off in a hot dry environment, you can use cotton for evaporative cooling. A cold wet t-shirt can dissipate significant heat. Be aware wet fabric on skin tends to chafe, so protect yourself from chafing. Choose a shirt that doesn’t rub, and use a lot of lube where it does touch the skin. A pre-frozen, ice soaked t-shirt worn strategically for a few hours can be a lot nicer than the pre-used sponge in the ice water bucket from the aid station. However, it’s going to feel damp and sticky eventually, which isn’t nice. Pack a back up tech shirt to change into when this happens.
Laundry
How to get the funk out of cotton: hot water. Cotton fibers wash and dry really well, they are stronger when wet, and once they are preshrunk, they can take very hot laundry temperatures which can kill the funky smell bacteria. Cotton can mildew if left rolled up in your gear bag. Once cotton has a case of mildew, it might never come out. Cotton fibers can also handle bleach well, but the dyes that are frequently used to color cotton don’t. The basic (high pH) properties of bleach breaks the bonds where the dye attaches to the fiber and you are left with bleach stained white spots that won’t go away.