From the same source: > Since some biospherians were starting to have symptoms like sleep apnea and fatigue, Walford and the medical team decided to boost oxyg

From the same source: > Since some biospherians were starting to have symptoms like sleep apnea and fatigue, Walford and the medical team decided to boost oxygen with injections in January and August 1993. The oxygen decline and minimal response of the crew indicated that changes in air pressure are what trigger human adaptation responses. These studies enhanced the biomedical research program. > [...] > The respiration rate was faster than the photosynthesis (possibly in part due to relatively low light penetration through the glazed structure and the fact that Biosphere 2 started with a small but rapidly increasing plant biomass) resulting in a slow decrease of oxygen. A mystery accompanied the oxygen decline: the corresponding increase in carbon dioxide did not appear. This concealed the underlying process until an investigation by Jeff Severinghaus and Wallace Broecker of Columbia University's Lamont–Doherty Earth Observatory using isotopic analysis showed that carbon dioxide was reacting with exposed concrete inside Biosphere 2 to form calcium carbonate in a process called carbonatation, thereby sequestering both carbon and oxygen. > [...] > After Biosphere 2's first mission, extensive research and system improvements were undertaken, including sealing concrete to prevent the uptake of carbon dioxide. Long story short: it was primarily an issue with the concrete absorbing the carbon dioxide (which otherwise would've been converted back into oxygen), lowering overall air pressure. Said issue was fixed by the second mission.