Altitude sickness is also known as acute mountain sickness. Altitude sickness can happen to anyone, no matter your fitness level, age or experience. It affects your breathing and your ability to take in oxygen. This can be life-threatening. It happens when you travel to a higher altitude too quickly, typically greater than 2500 m (about 1.5 miles) above sea level, often when mountain climbing, but also in cities that sit high above sea level.