The thiner air poses some medical threats to athletes, though. Each breath takes in less oxygen. Less oxygen in the lungs and blood stream can lead to mountain sickness. Training and play can be modified; however, to reduce negative effects. It can take up to eight days to adapt to the higher altitude. That's a huge plus to the Rockies and bane to the sea level Red Sox.
Update: FIFA banned international soccer matches in high altitude cities because of the impact on players. Some countries such as Bolivia are quite upset at this.