Altitude sickness affects people who aren't used to being in high altitudes. It's particularly common in mountainous regions. You might be affected in ski resorts or if you're an adventurous mountain climber. Cities like Aspen, Colorado, and Machu Picchu, Peru, are high enough to cause people to suffer from altitude sickness. Generally, altitude sickness strikes more than 60 percent of people who ascend to elevations of 8,000 to 10,000 feet (2,400 to 3,000 meters) or higher in a short period of time. The higher you go and the more quickly you go there, the more likely you are to suffer altitude sickness. Some people even feel altitude sickness at heights of 6,500 feet (2,000 meters); as a reference, Santa Fe, New Mexico, is located at 7,200 feet (2,100 meters).
Despite this, you aren't likely to feel any symptoms of altitude sickness on an airplane. That's because the cabins on planes are pressurized to simulate oxygen levels found at 8,000 feet or lower. The cause of altitude sickness is the thinner air found at higher elevations. With less oxygen in the air, your lungs take in less oxygen per breath than they're used to. This means you end up with lower levels of oxygen in your blood and you can end up with headaches, dizziness, fatigue, trouble sleeping, nausea and a loss of appetite. Slow ascents that include day-long, overnight stops every 1,000 feet (300 meters) make you less likely to suffer from altitude sickness. It's also important to remain well hydrated when ascending to new elevations.
If you end up with symptoms of altitude sickness, stay at your elevation until they pass. However, if you suspect you're coming down with more than the common altitude mountain sickness, and that you might have high-altitude pulmonary edema or high-altitude cerebral edema, descend to a lower altitude and seek immediate medical care.