The higher altitudes mean increased levels of harmful ultraviolet (UV) radiation compared to sea level areas. UV exposure increases 8-10% with every 1,000 feet above sea level. At an altitude of 9-10,000 feet, UV may be 45-50% more intense than at sea level. In addition, snow reflects about 80% of the UV light from the sun, meaning that you are often hit by the same rays twice. This only contributes to the problem, further increasing the risk for skin cancer. Also keep in mind that about 90% of UV rays can penetrate haze and clouds, so skiers and vacationers should not have a false sense of security.
The bottom line is that you should treat skiing just like a trip to the beach: apply a SPF15 or higher sunscreen on all exposed skin every two hours and wear 100% UV-blocking sunglasses or goggles.
"Essential Sun Safety Information for Skiers." Skin Cancer Foundation. January 2008. 18 January 2009.
"Skiers warned of skin cancer risk." BBC News. 18 January 2009.