Winter backpacking requires a higher level of skill and generally more specialized gear than in other seasons. Skis or snowshoes may be required to traverse deep snow, or crampons and an ice axe where needed. Winter sleeping bags and tents are essential, as are waterproof, water-repellent, and moisture dissipating materials. Cotton clothing retains moisture and chills the body, both particularly dangerous in cold weather. Winter backpackers stick to wool or synthetic fabric like nylon or polypropylene, which hold less moisture and often have specialized wicking properties to dissipate sweat generated during aerobic activities. Layering is essential, as wet clothes quickly sap body heat and can lead to frostbite or hypothermia.