Altitude Sickness occurs when people who are not acclimatised to high altitudes go quickly from lower altitudes to altitudes of 2,500 m (8000 ft) above sea level or higher.
Broadly, the level of risk is depends on the altitude you travel to, as well as the speed of your ascent:
Altitude sickness seldom occurs at this level or below.
The risk of acute mountain sickness (AMS) starts to increase at altitudes above 2,000 m.
Ascending to heights greater than 2,500 m puts you at possible risk of AMS.