Throughout the International Space Station (ISS) program, anecdotal reports have suggested that crew members 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 5,300 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.
Crew members are routinely asked about CO₂ symptoms in their weekly private medical conferences with their crew surgeons. In recent ISS expeditions, crew members have noted symptoms attributable to CO₂ starting at 3,000 ppm. Between 3,000–3,500 ppm, fatigue and full-headedness have been reported. Between 3,500–4,000 ppm, there have been self-reports of procedure missed steps or procedures going long. Above 4,000–4,500 ppm, headaches have been reported. A wide range of inter- and intra-individual variability in sensitivity to CO₂ has been noted.
Operational / Clinic relevance
These preliminary data provide semiquantitative ranges that have been used to inform a new operational limit of 3,500 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 2,600 ppm may maintain health and performance. Future work is needed to establish long-term ISS and future vehicle operational limits.