Why? Cotton does not dry easily, and once it is wet, not only does it have zero insulation; it actually steals heat from your body. That's when hypothermia sets in. A very dangerous,and not uncommon, situation.

Remember, jeans are usually made of cotton; plus they are bulky and heavy to pack. But okay .... if you MUST wear you jeans, go ahead and bring just one pair.

Of course, you can also buy and enjoy wearing cotton souvenir t-shirts while you are here, but don't wear them on any adventures outdoors!