If you live and work in a colder climate, your long underwear probably gets a lot of use, so durability is an important factor to consider. In general, synthetic base layers are more durable than those made of natural materials, like wool, but sometimes synthetic and natural fibres are combined to get the best of both worlds. You can also extend the life of your wool underwear by washing it less often, using cool or warm water and air drying it instead of using the tumble dryer. While synthetics are tear-resistant, they tend to become smelly over time and may need replaced before they are worn out due to odour.