Throughout the International Space Station (ISS) program, anecdotal reports have suggested that crewmembers develop CO₂-related symptoms at lower CO₂ levels than would be expected terrestrially. Since 2010, operational limits have controlled the 24-hour average CO₂ to 5300 ppm, or below as driven by crew symptomatology. In recent years, largely due to increasing awareness by crew and ground team, there have been increased reports of crew symptoms. The aim of this presentation is to discuss recent observations and operational impacts to lower CO₂ levels on the ISS.

Case presentation:

Crewmembers are routinely asked about CO₂ symptoms in their weekly private medical conferences with their crew surgeons. In recent ISS expeditions, crewmembers have noted symptoms attributable to CO₂ starting at 3000 ppm. Between 3000-3500 ppm, fatigue and full-headedness have been reported. Between 3500-4000 ppm, there have been self-reports of procedure missed steps or procedures going long. Above 4000-4500 ppm, headaches have been reported. A wide range of inter- and intra-individual variability in sensitivity to CO₂ have been noted.

Operational / Clinic relevance:

These preliminary data provide semi quantitative ranges that have been used to inform a new operational limit of 3500 ppm as a compromise between systems capabilities and the recognition that there are human health and performance impacts at recent ISS CO₂ levels. Current evidence would suggest that an operational limit between 650 and 2600 ppm may maintain health and performance. Future work is needed to establish long-term ISS and future vehicle operational limits.