The process of adapting to the decrease in oxygen concentration at a specific altitude. A number of changes must take place for the body to operate with decreased oxy- gen. These changes include increasing the depth of respiration; increasing the pressure in the pul- monary arteries, forcing blood into portions of the lung that are normally not used at sea level; manu- facturing additional oxygen-carrying red blood cells; and manufacturing extra 2, 4-DPG, a sub- stance that facilitates the release of oxygen from hemoglobin to the body tissues. Acclimatization generally takes 1 to 3 days and occurs after any significant altitude change above 1,220 meters (approximately 4,000 feet). Acclimatization is the body’s natural means of correcting altitude sickness and the rate of acclimatization depends on the alti- tude, rate of ascent, and individual susceptibility.