"He was wearing all cotton, which is the worst fabric for cold, wet weather. The weather just got the best of him," reads an official statement by Alaska State Troopers about the death of a hiker there in 2005. This is how and why cotton can kill you.
The Mayo Clinic definition reads: "Hypothermia is a medical emergency that occurs when your body loses heat faster than it can produce heat, causing a dangerously low body temperature. Normal body temperature is around 98.6 F (37 C). Hypothermia (hi-poe-THUR-me-uh) occurs as your body temperature passes below 95 F (35 C)."
Over 1,500 people in the US die from hypothermia each year. It causes a variety of things in your body to stop working, but the most important seems to be your heart. It will stop if your core temperature drops too low.
So you only have to worry about it in sub-freezing temperatures right? Wrong. While cold conditions will effect us all differently depending on our general health, physical fitness and other factors like genetics, hypothermia can be experienced in surprisingly warm weather. For instance, the Mayo Clinic warns that elderly persons may be subject to hypothermia, "…in an air-conditioned home."
Hikers are more likely to die of hypothermia in the spring, summer and fall than they are in the winter; during those months the odds that they'll be caught unprepared are simply higher.
And, the Mayo Clinic also specifically cautions against wearing cotton, saying, "Wool, silk or polypropylene inner layers hold body heat better than cotton does."
Cotton And Water
Cotton garments can absorb up to 27 times their weight in water, something which means they a) take forever to dry out and b) actively work to cool your body in even moderate temperatures.
And you can get cotton wet without exposing it to rain or submersion. Sweat heavily in it and it will soak up that sweat and hang onto it, which can lead to any of the problems described in this article as easily as falling into a lake will.
Why is cotton so absorbent? According to the Appalachian Mountain Club:
"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."
"Processed cotton fibers are 99 percent cellulose. Cellulose is a polymer composed of a long chain of connected glucose molecules that each contains three hyrodoxol groups with slight negative charges. Water, as you may remember from high school chemistry, has a slightly positive charge (the oxygen atom draws in the two hydrogen atoms' electrons). The upshot is that water molecules are attracted to—and bond with (via hydrogen bonds)—the zillions of hydroxol groups in cotton. This, coupled with the vast amount of space contained within and between the fibers, provides cotton with its tremendous water-absorbing properties."
Cotton traps water inside its fibers, which is why it takes so long to dry out.
Why Being Wet Is A Problem
According to the United States Search and Rescue Task Force, "Water conducts heat away from the body 25 times faster than air because it has a greater density (therefore a greater heat capacity). Stay dry = stay alive!"
Getting wet and staying wet, even in above-freezing temperatures can rapidly cool your body as a result.
And, by filling up with water, an insulating cotton garment such as a hoody loses the trapped air space that made it warm when it was dry. This exposes you more significantly to simple radiant heat loss, where the shear difference in temperature between your body and the environment causes you to get colder.
You'll also be more subject to evaporative heat loss as water slowly leaves the cotton garment, something exacerbated by wind chill.
All together, getting wet and staying wet outdoors is simply a recipe for disaster.
How Other Fabrics Keep You Warm When They're Wet
Wool: The outer layer of each wool fiber is a filmy skin called an epicuticle. This coating repels water drops, preventing them from soaking through the fibers. Due to the fuzzy nature of wool fabrics, rain droplets are less likely to break up, instead beading on the surface and running off. When water is in vapor form from humidity or sweat, it can pass through the epicuticle and be absorbed into the wool fiber; each fiber can absorb up to 37 percent of its weight without feeling wet, pulling water vapor away from your body and spreading it across a larger surface area so it can evaporate. And finally, the "crimp" or kinkiness of the wool fibers also builds dead air space into any wool garment, providing insulation even when wet.