During the two year mission when the eight member crew was sealed into the structure, oxygen did deteriorate.
Oxygen began at 20.9%, but over 16 months fell to 14.5%.
This is the equivalent of oxygen at about 4,000 meters (13,000 feet).
Some of the biospherians were starting to have symptons of oxygen deprivation such as sleep apnea and fatigue. The outside medical team decided to make two injections of oxygen into the structure during the last few months of the two year mission.
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