Final answer:
The environment in which athletic performance declines significantly for a large percentage of people, even after acclimation, is high altitude. Acute exposure to high altitudes results in low oxygen levels in the air, leading to a condition called acute mountain sickness (AMS). Through the process of acclimatization, the body adjusts to the high altitude over time, increasing the production of red blood cells to transport available oxygen.
Step-by-step explanation:
The environment in which athletic performance declines significantly for a large percentage of people, even after acclimation, is high altitude. Acute exposure to high altitudes results in low oxygen levels in the air, leading to a condition called acute mountain sickness (AMS). Through the process of acclimatization, the body adjusts to the high altitude over time, increasing the production of red blood cells to transport available oxygen.