High altitude: Altitude sickness occurs at high altitude. So what is high altitude? Altitude is defined on the following scale: High altitude: 8,000 - 12,000 feet (2,438 - 3,658 meters); Very high altitude: 12,000 - 18,000 feet (3,658 - 5,487 meters); and Extremely high altitude: 18,000+ feet (5,500+ meters). Most people can go up to 8,000 feet (2,...