Altitude sickness is a condition caused by the lower levels of oxygen found at high elevations. When you travel from a low altitude to a high one, your body needs time to adjust to its new environment. The most common form of altitude sickness is acute mountain sickness. Since your lungs aren't taking in as much oxygen per breath as they usually do, the oxygen level in your blood drops and you can end up dizzy, fatigued, nauseated or with headaches and trouble sleeping. However, acute mountain sickness normally passes a few days after you reach your high-altitude destination. The key to properly coping with acute mountain sickness is to stay well hydrated and to remain at the same altitude until your symptoms pass.
Whether you'll get altitude sickness when you go skiing depends on a few factors: what elevation you're coming from, what elevation you're going to, how you prepared and luck of the draw. Normally, acute mountain sickness doesn't kick in until you're about 8,000 feet above sea level (2,400 meters). However, some people can feel symptoms at altitudes of 6,000 feet (1,800 meters). The bigger the difference between where you've come from and where you're going, the higher your chances are of having altitude sickness. Speed also makes a difference: The quicker you change altitudes, the more difficult it is for your body to adjust. You need to take it easy and allow your body to acclimate; drink a lot of water even before you arrive, rest and avoid caffeine and alcohol -- at least at first. Don't hit the slopes right away, especially if you feel acute mountain sickness coming on.
To get a sense of the height of ski slopes, consider that Vail, Colorado, is located at 8,000 feet (2,400 meters) above sea level, and its highest slope stretches to 11,570 feet (3,526 meters). New York City is 410 feet above sea level at its highest point (125 meters).