The phrase “cotton kills” is an old adage in outdoor recreation circles. Though it might sound dramatic, this statement is actually supported by science. Cotton greatly increases the risk of hypothermia for outdoor recreators because a) it absorbs significantly more water than other fabrics, and b) loses the ability to insulate when wet. Why? It all has to do with how cotton is structured.

This excerpt from a blog post by the Appalachian Mountain Club explains how cotton traps water:

“A cotton fiber is like a tiny tube formed of six different concentric layers. As individual cotton fibers grow on the plant, the inside of the “tube” is filled with living cells. Once the fiber matures and the cotton boll opens up to reveal its puffy white contents, these cells dry up and the fiber partially collapses, leaving behind a hollow bean-shaped canal, or “lumen”. This empty space holds lots of water.”

As a result of this empty space, cotton is able to absorb up to 2700% of its weight in moisture. For comparison, wool only absorbs about 30% of its weight and synthetic fibers, like nylon and polyester, soak up little to none.

Two hikers and a dog stand on iceThis is important because dry fabric is better at keeping you warm. By trapping air against your skin, dry fabric creates an insulating “microclimate”. Wet fabric, on the other hand, sticks to your skin and evacuates air away from the body. Additionally, as wet fabric dries, it takes heat from your body in a process called evaporative heat loss. Given that water conducts heat away from the body 25 times faster than air, this puts you at great risk for developing hypothermia, especially if outside temperatures are cool or sub-freezing.

To prevent this from happening, many synthetic fabrics, such as polyester and nylon, are designed to perform a “wicking” action, which moves moisture from your base layers to your outer layers, where it can evaporate without drawing heat from your body. This is another area where cotton fails. Because of the aforementioned lumens, which trap water, cotton is structurally unable to wick moisture away, and will instead hold it against your body. Note that this applies to all forms of cotton, including corduroy, denim, and flannel. Clothing made from rayon, bamboo, silk and synthetic silk-imitations also absorb water and fail to properly wick it away from the body.

So, with this all said, what fabrics work best for minimizing hypothermia risk? There are three, which we have mentioned a few times here: wool, polyester, and nylon. Wool, the only natural fiber on this list, maintains its insulating properties even when wet, though it does absorb some water. Polyester and nylon are both synthetic fabrics, making them successful at repelling moisture and wicking it away from your skin. For your safety, your clothing layers should be composed of these fabrics.