Maybe you're an avid extreme winter mountain trekker or you simply get the chills while working in the cool office, you operate heavy mining equipment year round or you work for the US Coast Guard. Either way, thermal long underwear, also known as base layer or long johns, may be just what you need to stay warm and comfortable. The type you choose will depend on your activity level, your budget, and the temperature.There are many types of thermal underwear but our customers usually choose one of these top three.Merino WoolMerino Wool thermal underwear is our warmest underwear. It doesn't itch or smell like regular wool can. It is very soft and comfortable against your skin. It wicks moisture away from your skin when you get wet. It also maintains its thermal properties, keeping you warm, even when it is wet. Best of all, it is made from natural materials that are environmentally sustainable. Merino sheep need a haircut once in a while just like you do - no harm done.PolypropylenePolypropylene is a popular material for thermal underwear because of its affordability. It is less expensive than many other options, but has many of the same benefits. Polypropylene will stay warm when it gets wet. It is durable and will last a long time. It dries very fast compared to other materials and it also wicks moisture away from the skin.100% Cotton100% Cotton thermal underwear and long johns are hands down some of the most comfortable, warm underwear you can get. It is very affordable, comes in a variety of colors and styles, and is made from renewable and natural fibers. The comfort and feel of cotton is hard to beat.