Cotton is consistently crapped on in the outdoor community for backpacking clothing and other outdoor activities. “Cotton kills” is a common saying among outdoor enthusiasts and anyone in the backcountry in jeans might as well have noob written on their forehead. But recently I’ve seen the death throes of cotton advocates citing that it is a perfectly acceptable and suggested fabric for desert environments. I am open to more arguments but after reading some of the articles justifying the reasoning, I have to say I’m very skeptical of this opinion. In this post, I want to explain why cotton gets such a bad rap, provide some explanation for why synthetic fabrics are preferable in any environment, and explain why cotton sucks even in the desert.

Les Stroud is famous for repeating his catchphrase, “you sweat, you die,” throughout his show Survivorman, where he demonstrated an incredible ability to handle the elements with limited gear in foreign environments and escape safely. However, as he is well aware but ignored in his simple statement, sweating is a brilliantly evolved survival mechanism to keep our core body temperature stable even in temperatures far exceeding the temperature of the surrounding environment. This summer, hiking in Death Valley in temperatures exceeding 118 degrees Fahrenheit, the sheer brilliance of this amazing evolutionary trait truly became apparent to me. How is it that my sealed bottle of water equilibrated with the atmospheric temp and yet I managed to remain nearly 20 degree cooler than that?

The primary reason in this situation is because of evaporation, a mechanism which reduces the average temperature of a body of fluid by expelling the highest energy molecules. Temperature is a measurement of average kinetic energy of a substance. However, as with anything, some molecules exceed that average, and some are below it. The highest energy particles, when they reach the junction of the liquid and a gas, sometimes have enough energy to escape the liquid altogether and become gaseous. And when the highest energy particle is expelled, the average drops. Say you have molecules with effective “temperatures” of 97, 98, 98, 99, 99, 99, 100, 100, and 101. The average temperature of that body is a very slightly elevated 99 degrees. The body kicks off the 101 degree molecule via evaporation and the temperature drops to a healthier 98.75 degrees. Simple but effective, and that is how your body can keep from cooking in ridiculously hot temperatures.

However, the cooling mechanism of sweat depends on the water actually actively evaporating to reduce the average energy. When sweat drips off or is absorbed into clothing it doesn’t confer the advantages of evaporation. Sweat dropping to the ground reduces the total energy of the body, but the average temperature remains the same. The same thing happens with sweat that is absorbed into clothing. As we are all well aware from the consistent use of cotton in towels and wash cloths, it is incredibly good at absorbing water. Something else you may have noticed is that wet towels take ridiculously long to dry. This is because the actual fibers in the cotton are clinging on to the water molecules like long-lost lovers and it takes significantly more energy to tear these two lovers apart. This requires your body temperature to rise even higher and donate even more energy to these molecules before they can do their job of turning into a gas.

This is where synthetic fabrics come in. The plastic fibers in synthetic fabrics don’t absorb water, but rather “wick” it toward the surface where it can easily evaporate. Not only that but they actually facilitate the process. Here’s how. As I noted above, evaporation depends on a intersection of liquid and gas to occur. The more surface area where the liquid is exposed to air, the more chances a high energy molecule has to say its farewells and enter the atmosphere. All the little intertwined threads in synthetic fabrics have a surface area much, much larger than your skin for the sweat to spread out on and thus actually enhance the evaporative cooling effect.

So why do people still think cotton is beneficial in hot environments?

• The predominant reasoning that I have consistently seen in researching why cotton may be preferable in desert environments is that it holds the water on your skin and slows the rate of evaporation, cooling you for longer. Unfortunately, while seemingly good reasoning, it doesn’t translate into an effective advantage. A healthy individual pumps out sweat at the ideal rate to maintain the body’s temperature. Any hindrance to that will not help it but will rather work against the cooling effect.
• Another one of the reasons that I have been entertaining is that cotton helps absorb the otherwise useless beaded and dripping sweat in humid environments and facilitates evaporating it at a rate more manageable with the environment. But anyone who has worn super lightweight polyester clothing knows that it acts equally well in this regard. It holds the sweat and distributes it but unlike cotton, it is only the weave of the fabric that loosely hangs onto the water, not the fibers themselves.
• I’ve also read the argument that loose-fitting polyester clothing will facilitate evaporation at the surface of the clothing, not on the skin. But this seems more of an issue of fitting than fabric itself. Theoretically tighter fitting clothing would be more effective, independent of the type of fabric.
• And lastly, I have read the belief that a faster rate of evaporation will increase the amount that you sweat. Superficially, this seems like one of those hair growing faster when you cut it myths. Additionally, from an anecdotal perspective, I know I am fully capable of sweating out liters of sweat while fully immersed in the pool during swim workouts and this completely goes against that hypothesis. From a biological perspective, I cannot find any evidence for a moisture sensing receptor on skin which could signal effectiveness of evaporation. However, even if it were true, ideally our core body temperature and hydration status dictate our rate of sweating. As we have discussed, sweating is an extremely effective mechanism for keeping the body cool. Granted adequate hydration and electrolyte replacement, sweating is extremely beneficial and should not be inhibited in hot climates.

But why is cotton, despite its ability to slow cooling in warm climates, harshly condemned even for colder temperatures? Namely, why isn’t cotton’s property of evaporation inhibition an effective strategy for keeping warm? Inhibiting evaporation actually is a very effective tactic for keeping warm and is the basis for vapor barrier socks and sleeping bag liners. Andrew Skurka has a good summary of their use here. However, an independently unfortunate property of cotton is that it loses all insulation when wet. Insulation acts by trapping air in dead space in a garment. It slows the loss of heat by warming and withholding the air in the garment rather than warming the transient air outside. While all fabrics lose some insulative properties when wet, synthetic fabrics more or less maintain this dead space and cotton does not, eliminating its ability to insulate and making it a poor choice for winter climates as well.

So cotton still sucks, in any environment. If you have an alternative hypothesis for why cotton may be preferable to an ultra thin tight-fitting polyester shirt in desert environments, please feel free to comment below with your explanation! There’s very likely something that I have overlooked and I’d be interested to hear your reasoning.