Clothing keeps you warm by trapping warm air near your skin. That’s how cotton kills. When cotton gets wet, it ceases to insulate you because all of the air pockets in the fabric fill up with water. When you hike, you perspire, and any cotton clothing touching your skin will absorb your sweat like a sponge.
If the air is colder than your body temperature , you’ll feel cold because your cotton clothing is saturated and no longer providing any insulation. This can lead to disorientation, hypothermia, and potentially death if you become too chilled. Remember, hypothermia can occur in temperatures well above freezing and become serious if you get wet and chilled.
Avoid wearing garments that are labelled as corduroy, denim, flannel, or duck. These are all made with cotton. In addition, steer clear of cotton-polyester blends, for example 50/50. They’ll still kill you, although it may take a little longer.