The Headache of Carbon Dioxide Exposures

Carbon dioxide (CO₂), a natural product of human metabolism, accumulates quickly in sealed environments when humans are present, and can induce headaches, among other symptoms. Major resources are expended to control CO₂ levels to concentrations that are tolerable to the crews of spacecraft and submersible craft. It is not practical to control CO₂ levels to those found in the ambient environment on earth. As NASA looks ahead to long-duration missions conducted far from earth, difficult issues arise related to the management and effects of human exposure to CO₂. One is the problem of “pockets” of CO₂ in the habitat caused by excess generation of the gas in one location without a mechanism to purge the area with fresh air. This results in the crew rebreathing CO₂ from their exhaled breath, exposing them to a much higher concentration of CO₂ than whole-module measurements would suggest. Another issue is the potential increased sensitivity to CO₂ in microgravity. For example, based on anecdotal information, it appears that space crews may be more susceptible than submarine crews to some of the subtle, yet adverse effects of CO₂ exposure. Another issue, not unique to spaceflight, is the possibility of inter-individual differences in the susceptibility of crewmembers to CO₂ exposure. Again, anecdotal reports from the International Space Station (ISS) crews suggest that certain individuals may experience a greater susceptibility. The implications associated with these issues are extremely important as NASA sets CO₂ exposure limits that protect the crew from this compound's subtle adverse effects, without causing an unwarranted expenditure of resources to scrub CO₂ from the habitat atmosphere.