Training at simulated altitude is comparable to training in the mountains. Mountain air contains less oxygen than air at sea level. At sea level, air contains 20.9% oxygen, and at 2500 meters the oxygen is down to 15.5%.
How it works is fairly simple. You inhale air that contains less oxygen which triggers the body. Inside the kidneys are sensitive cells that react to the lack of oxygen. They stimulate the production of erythropoietin. This hormone stimulates the production of red blood cells in the bone marrow.
These red blood cells transport oxygen through the body, this gives the muscles and organs more oxygen. More oxygen means a better circulation and improvement of metabolism.
For athletes and other target groups the advantages are: