Acute mountain sickness is caused by reduced air pressure and lower oxygen levels at high altitudes. The faster you climb to a high altitude, the more likely you will get acute mountain sickness. The best way to prevent altitude illness is to ascend gradually. It is a good idea to spend a few days ascending to 9850 feet (3000). Above this point ascend very slowly so that the elevation at which you sleep does not increase more than 990 feet to 1640 feet (300m to 500m) per night.