I have upgrades to merino wool long underwear for elk hunting. For 25 years I used polypropylene long underwear (initially for backpacking -- I didn't start big game hunting until 2002 and elk hunting in 2006). The poly worked but started to get a little holy and scratchy. I got medium weight Minus 33 merino wool and love it. It is comfortable next to my skin. It breathes and releases sweat well. It is warm. Love it. I hunt elk at 10,500 feet in SW Colorado in mid-October. It is cold in the mornings but warms up through the day if the sun is out (15 degrees in the morning, 55 degrees in the afternoon). I think I have medium weight merino wool, which is suitable to our elk hunting temperature ranges.