Why hikers say “Cotton Kills”

And why wool and Polyester are better for backpacking


Have you heard hikers recite the phrase “Cotton Kills” with a sort of blind devotion reminiscent of a fully brainwashed member from a religious cult? Have you ever wondered what the heck they were talking about? In this post, we’ll look into the thinking behind that blanket statement, examine the why cotton is a fabric that is dangerous to use in the backcountry. We'll also look at what fabrics are most ideal to wear and why.


At some point, I’d like you to conduct an experiment: Wear a pair of sneakers (of the non-waterproof variety) for a walk in the pouring rain. On one foot, wear a cotton sock. On the other, wear a merino wool hiking sock of a similar thickness. Don’t forget to also wear layers and rain gear appropriate to the conditions! Walk outside in the rain until your shoes are fully soaked.

You should notice within a few seconds of your shoes and socks becoming saturated that both feet become immediately cold and uncomfortable. Be strong. Keep walking. In less than 10 minutes, you should notice the foot with a wool sock beginning to feel warmer and more comfortable. The cotton sock will still feel bone-chillingly cold. Eventually, the wet foot in wool should feel just fine, like you could hike with that wet wool sock for several more hours, while the cotton-clad food will feel like it is close to freezing.

Next, return home, dry off and warm up. Wring out the wet socks and hang them near one another. Check them every hour to observe their drying times. The wool sock should be nearly dry in about 2-3 hrs depending on its thickness, temperature, and humidity. The cotton sock will take considerably longer.

Am I right?

Think how much colder the foot encased in the wet cotton sock felt. While not life-threatening, even in temperatures well above freezing, you still risk trench foot by walking around all day in cold, wet footwear. This is just your feet. What about somewhere more vital to your body. What if you were wearing a cotton t-shirt and got caught in an unexpected rain shower. That cold cotton shirt stuck to your core, sucking heat from your body could lead to the early stages of hypothermia even on a warm summer day. It is critical to your survival that your clothing can keep you warm—or at least help retain some heat once it is wet, and that it be capable of drying quickly so it can return to full warmth more quickly. 

But why is wool warm when wet?

The marketing departments would have you believe that both wool and polyester are “warm when wet”. That statement, as you learned in the experiment above is bullshit. Wet clothing is always colder than dry clothing. But the cotton was unbearably cold compared to the wool. Clearly, each fabric has dramatically different properties. But why is this? It comes down to the structure of the threads in the fabric and how they react when they come in contact with water.


Wool fibers are complex in their structure. If you want to get super-nerdy, there are multiple layers to the fiber made up of cortical cells wrapped in cuticles, covered in a scaly layer, followed by another layer that has pores which assist in moisture transfer and temperature regulation in humid environments. However the most important structural feature is the natural crimp to wool fibers. These crimps help trap pockets help the garment trap microscopic pockets of air. When the fiber comes in contact with the water, it is able to retain its structure better than cotton without collapsing. This preserves tiny pockets of air, helping reduce heat loss. Additionally, because of all the crimps, there is less surface area of the garment actually in contact with your skin. When wet, this actually helps keep the warmth-robbing wetness off your skin. While the garment will be noticeably colder when wet than dry, it will never be that bone-chilling cold that cotton is famous for. Similarly, because the fibers don’t collapse, the garment dries much quicker, allowing you to get back to normal warmth within 1-3 hours depending on conditions.

Wool baselayers typically dry fully within 45-60 minutes while being worn on a sunny day (70°F, 20% humidity). Hanging wet clothes may paradoxically cause them to take longer to dry as they won't have your body heat helping to speed the process. Socks and thicker layers may take longer. Typically wet wool does not result in a clammy feeling as it dries, unlike polyester items. Wool is also naturally antimicrobial, meaning you can wear it for several days without developing that funky hiker smell.


Synthetic fabrics, such as nylon and polyester and polypropylene, also have threads that retain structure when wet. In fact, these are actually hydrophobic, meaning that while the water will wick into the fabric, it is held entirely in the matrix of the weave by surface tension and never absorbed by the material.

Synthetic materials are slightly warmer when wet than wool, and they tend to dry about 20% faster. They are also slightly warmer for their weight and usually much more affordable. The downside to synthetic clothing is that it can develop a powerful odor very quickly.

To combat the odor issue, manufacturers treat items like baselayers with anti-odor treatments. They use terms like “permanent odor protection”, but what they’re trying to say is that the odors are “reduced”. After a day of heavy use, you’ll begin to notice a brewing funk. On an untreated item, you’ll notice a much more potent funk after a mere 40 minutes. I started backpacking with all wool base layers and now use all synthetic. Yes, I am more smelly, but as card-carrying hiker trash, performance and weight is the most important metric.


Cotton, on the other hand, is great at absorbing water, and it will draw the water completely into the fabric. Because it lacks crimps like wool, the the wet cotton's structure collapses under the weight of the water. It loses the tiny insulating pockets of air, and allows the damp fabric to sit tightly against your skin where the moisture can rob you of warmth. In wet cotton clothing, you will rapidly lose your body heat, and if you are unable to swap out your wet clothes for dry ones, you’ll have a long wait before the wet layers dry. It can take hours for cotton to dry. In practice, the trail rarely warm enough, dry enough (rain or humidity) to dry out wet cotton. Once wet, cotton will likely remain that way for the rest of the trip.

Cotton Blends

A new entrant to athletic wear for non-hiking activities, cotton blends weave in some percentage of synthetic fibers, promising faster drying times. They usually have fancy high-tech sounding names courtesy of the weenies in the marketing department like “Fast Dry Cotton”. Don’t believe the hype. While these items will dry significantly faster than standard cotton, they will still be colder when wet and still take longer than wool or pure synthetics. Sure cotton is cheap and feels amazing when it's dry. But once wet, you’re better off with smelly poly or pricey wool.


When you venture away from the protections of civilization, you must become more responsible for your own comfort and survival. Choosing the right materials for your clothing and equipment is essential. Cotton, while fine for athletic use within civilized areas, becomes a gigantic risk out on the trail. When attempting to stay dry, you’re battling not only the elements but also your own sweat creation. It’s simply too dangerous to wear clothing that chills you when wet, and effectively never dries once it gets wet. With cotton, one mistake with moisture management and you could be headed for a stroll down hypothermia lane. Having been once, I can assure you, is not a fun place to visit.

Next steps:

  • Learn how to put together an effective layering system for 3-season hiking and backpacking.
  • Learn about the four mechanisms that govern heat loss and how they apply to your clothing and equipment.
  • Deciding between wool and polyester? Check out the head to head comparison of the two fabrics.

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