Cotton is one of the most popular fabrics in the world: cheap, soft, and breathable, which is why so much of what we wear, down to our sleepwear, is made of it. So why do experienced hikers repeat the grim little saying “cotton kills”?
The short version: cotton’s poor moisture management, slow drying, and loss of insulation when wet make it a bad and sometimes dangerous choice for hiking and other hard, sweaty activity, especially in cold, wet, or humid conditions. I ditched cotton from my hiking wardrobe years ago, down to socks, underwear, and pants, and below I will explain exactly why, with the science behind it, the one situation where cotton helps you, and what to wear instead. If you want the full clothing system, see my guide on what to wear hiking.
The Science: Why Cotton Fails on the Trail
It Soaks Up Water and Holds It
Cotton is hydrophilic, or water-loving. Its fibers pull in moisture from sweat, rain, or humidity and hold it against your skin, and cotton can absorb roughly 25 to 27 times its own weight in water. A performance fabric like polyester is the opposite: it is hydrophobic and holds almost no water (its moisture regain is a fraction of a percent), so it moves sweat to the surface to evaporate instead of soaking it up.
Hiking is sweaty work. I start sweating after just two to three hours of steady hiking even in mild weather, and some people sweat far more. In cotton, all of that sweat goes straight into the fabric and stays there.
Related: Best Material for Hiking Socks
It Dries Painfully Slowly
Because cotton holds so much water, it dries very slowly, especially in humid air, shade, or low wind. A synthetic or wool shirt can feel dry again in the time it takes cotton to still be clammy and cold. When I am backpacking, I cannot afford to wait around for clothes to dry before moving on, which is reason enough on its own to leave cotton at home.
It Stops Insulating the Moment It Gets Wet
This is the dangerous part. Dry cotton insulates by trapping air in its fibers. When it gets wet, water fills those air pockets and the insulation collapses, so instead of keeping heat in, wet cotton starts pulling heat out of you.
Water conducts heat away from your body far faster than air does. Research from the U.S. Army has been widely cited for the finding that wet clothing can pull heat from the body many times faster than dry clothing, often quoted as up to about 25 times. Your body normally loses only a small share of its heat through direct contact (conduction), but against cold, wet cotton that share climbs sharply. Add evaporative cooling as the water slowly leaves the fabric, plus wind, and you have a fast track to being dangerously cold.
It Chafes
Once it is soaked, cotton stops feeling soft and turns abrasive. Wet, heavy fabric rubbing on skin causes friction, and that means chafing, a stinging red rash in the usual spots (inner thighs, underarms). Chafing is common enough that studies of endurance athletes put it in the double digits, and on a long day it can end a hike as surely as a rolled ankle. My guide on how to prevent chafing goes deeper, but step one is not wearing cotton.
What to Wear Instead
The goal is a fabric that moves moisture away from your skin, dries fast, and keeps insulating even when damp. Two families do this well.
Synthetics (Polyester and Nylon)
Polyester and nylon are the workhorses of hiking clothing. Because they are hydrophobic, they wick sweat to the surface where it evaporates, so they dry fast and barely hold water. They are also lightweight, durable, stretchy for movement, and relatively affordable.
If you’re replacing cotton and want specific options rather than just fabric advice, my hiking shirt comparison covers the synthetic tees, merino shirts, sun hoodies, and vented button-ups I would actually consider for the trail.
Most of my hiking shirts and many of my socks are polyester or nylon. The one real downside is odor: synthetics trap smell more than wool does, though still far less offensively than sweat-soaked cotton. If you want the deeper dive, see is polyester good for hiking. For trail use, I generally prefer purpose-built hiking socks made from merino wool or performance synthetic blends. For the lower body, my tested hot-weather hiking pants show how different synthetic weaves actually feel once you start sweating.
Merino wool
Merino wool is my favorite for next-to-skin layers, because it does the one thing cotton cannot: it keeps insulating even when wet. Each wool fiber has an outer layer that sheds water while the core manages moisture, so merino can absorb a good share of its weight in moisture and still feel dry and warm. It also regulates temperature well and resists odor remarkably.
On a 5-to-7-day trip I carry two pairs of socks: one merino, one mostly polyester as backup. The merino pair I can wear for days without them stinking, and I rarely even touch the backup. Try that with cotton socks and you would not want to put them back on the second morning. The trade-offs are that merino costs more and is less durable on its own, which is why it is usually blended with a little nylon for strength. It is worth it for socks and underwear especially. For the full fabric comparison, see the best material for hiking socks.
A Note on Blends and Layering
Watch out for cotton blends. A “50/50” cotton-poly shirt still holds a lot of water and dries slowly, so it carries most of cotton’s problems. The most important rule is your base layer, the piece against your skin, should never be cotton. Build from a wicking synthetic or merino base, add an insulating mid-layer, and top with a waterproof shell, and you have a system that handles sweat and weather instead of trapping them.
When Cotton Is Fine
Cotton is not evil, it is just the wrong tool for hard, sweaty, or cold-and-wet activity. It is perfectly fine for short, warm-weather walks close to home, casual wear, and lounging at a warm, dry camp. And as covered above, a deliberately dampened cotton shirt can help you in hot, dry desert heat. The problems show up on longer hikes, in the cold, in the wet, and anywhere conditions can change on you, which is most real hiking.
Summary
Cotton is comfortable and cheap, but it is a poor and sometimes dangerous hiking fabric because it soaks up water, dries slowly, and stops insulating the moment it gets wet, which drains your body heat and can lead to hypothermia even in mild weather. It also chafes once it is soaked.
For the trail, choose moisture-wicking synthetics (polyester, nylon) and merino wool, keep cotton out of your base layers, and save cotton for casual, warm, dry settings, or for the rare desert day when you want a wet shirt to cool you down. Get the fabric right and you stay drier, warmer, and far more comfortable from the first mile to the last.
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