A balaclava (pronounce /ˌbæləˈklɑːvə/), also known as a balaclava helmet or ski mask, is a form of headgear covering the whole head, exposing only the face or upper part of it, and sometimes only the eyes. The name "balaclava" comes from the town of Balaklava in Crimea. During the Crimean War, knitted balaclavas were sent over to the British troops to help protect them from the bitter cold weather. They are traditionally knitted from wool, and can be rolled up into a hat to cover just the crown of the head. Modern balaclavas can be made from a number of materials, such as silk, cotton, polypropylene, neoprene, wool or fleece. Modern balaclavas are also used in outdoor winter sports activities such as skiing, snowboarding, or winter bicycling to help protect the face from the cold wind and maintain warmth. Some exercise specialists believe that balaclavas help athletes with exercise-induced asthma by recirculating exhaled moisture (in concurrence with the theory that it is the dryness of air, not temperature, that triggers some types of asthma). Motorcyclists also wear one under their crash helmets for similar reasons; balaclavas also help to keep the inner lining of the helmet clean.