So, 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 aren’t my running shirts cotton?
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 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 fibers that are measured in longer units such as yards/meters and miles/ kilometers. A cotton boll produces fibers that are only a few inches long. They are spun together to make the yarn, and then typically knitted together to make the cloth that is used in T-shirts. Cotton is naturally absorbent and breathable, which feels great against the skin when dry. However, cotton loves to hang onto that water it absorbs and it tends to stay wet. Therefore, when we sweat it holds onto that sweaty funk, and 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 and skin irritation. However, if you are looking for something to help cool you off in a hot dry environment, using evaporative cooling you can use a cold wet t-shirt to your advantage. Be aware wet fabric on skin tends to chafe, so protect yourself from chafing, i.e. use a lot of lube and make sure the shirt is cut in a way to avoid rubbing. 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. But, it’s still going to feel damp and sticky against your skin, which isn’t as nice as your typical tech running shirt.
Getting the funk out: 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 help with that funky smell. High temperatures kill the bacteria that cause the funk. Be cautious about leaving it rolled up in your gear bag post race though, 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.
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