What does oxygen become at high altitudes in the atmosphere?

Oxygen becomes "thin." As in it becomes more scarce, and it is harder for a person's body to take in and absorb enough oxygen. This is why people become easily winded at high elevations, and can even require oxygen tanks when at an elevation high enough that there is not enough oxygen in the air for the person to breathe. Example:  The peak of Mt. Everest

Migration people get up and go cause they feel it is best for them 

Answerstratovolcanoes///near or in the ring of fire;