Ditto, natural fibers, cotton or wool, next to skin. I don't want anything next to skin under other clothes that could pose a threat to my life. The one exception to the natural fiber rule is nomex. Nomex has some very good attributes and like wool, it doesn't support flame, it just chars. Massif HotJohns long underwear is good stuff, comfortable and cleans just like polyester (hang dry). Massif nomex fleece is good outerwear too.