Crew Health and Performance Improvements with Reduced Carbon Dioxide Levels and the Resource Impact to Accomplish Those Reductions

Abstract

Carbon Dioxide (CO 2 ) removal is one of the primary functions of the International Space Station (ISS) atmosphere revitalization systems. Primary CO 2 removal is via the ISS's two Carbon Dioxide Removal Assemblies (CDRAs) and the Russian Carbon Dioxide removal assembly (Vozdukh); both of these systems are regenerable, meaning that their CO 2 removal capacity theoretically remains constant as long as the system is operating. Contingency CO 2 removal capability is provided by Lithium Hydroxide (LiOH) canisters, which are consumable, meaning that their CO 2 removal capability disappears once the resource is used.With the advent of 6 crew ISS operations, experience showing that CDRA failures are not uncommon, and anecdotal association of crew symptoms with CO 2 values just above 4 mmHg, the question arises: How much lower do we keep CO 2 levels to minimize the risk to crew health and performance, and what will the operational cost to the CDRAs be to do it? The primary crew health concerns center on the interaction of increased intracranial pressure from fluid shifts and the increased intracranial blood flow induced by CO 2 . Typical acute symptoms include headache, minor visual disturbances, and subtle behavioral changes. The historical database of CO 2 exposures since the beginning of ISS operations has been compared to the incidence of crew symptoms reported in private medical conferences. We have used this database in an attempt to establish an association between the CO 2 levels and the risk of crew symptoms. This comparison will answer the question of the level needed to protect the crew from unacceptable risk of acute effects. As for the second part of the question, operation of the ISS's regenerable CO 2 removal capability reduces the limited life of constituent parts. It also consumes limited electrical power and thermal control resources. Operation of consumable CO 2 removal capability (LiOH) uses finite consumable materials, which must be replenished in the long term. Therefore, increased CO 2 removal means increased resource use, with increased logistical capability to maintain necessary resources on board ISS. We must strike a balance between sufficiently low CO 2 levels to maintain crew health and CO 2 levels which are operationally feasible for the ISS program. Nomenclature BMD

Relationship Between Carbon Dioxide Levels and Reported Headaches on the International Space Station

“…Although it is desirable to minimize CO 2 exposure from a human health standpoint, it is not currently practical to limit ppCO 2 to below 3 mm Hg with six crew members using the suite of hardware. 17 To study the effects of CO 2 on the ISS more rigorously, a working group was formed as a multiyear collaboration between the Astronaut Occupational Health Program, Space Toxicology Office, and Space and Clinical Operations at the NASA Johnson Space Center. The objectives of this study were to comprehensively characterize CO 2 levels since the beginning of the ISS program, identify in-flight headache events, and analyze the relationship between ppCO 2 and headache.…”

Prediction of Crew Health Effects from Air Samples Taken Aboard the International Space Station

“…Headaches are occasionally reported by the crew during private medical conferences (PMCs) and these can be associated with concentrations of CO 2 that approach the index value. If the average concentration of CO 2 during the week before a PMC is about 5 mmHg, then there is a 4% risk that a crewmember will report a headache during that PMC ( 5 ). If the weekly concentration has averaged about half the index value (2.5 mmHg), the risk of a headache being reported during a PMC is about 1%.…”

Surprising Effects of CO2 Exposure on Decision Making

“…Preliminary data suggest that there is approximately a 4% risk that a headache will be reported by a crewmember during a private medical conference if the CO 2 levels have averaged about 4 mmHg during the previous day. 2 It is conceivable that CO 2 is linked to the "space stupids" phenomenon reported by astronauts. 8 On occasion crewmembers are unable to complete tasks in space that were relatively easy for them to perform on the ground.…”

“…Even aboard the International Space Station (ISS), considerable resources are necessary to maintain operational levels of CO 2 near 2 mmHg with a 6 person crew. 2 The situation within an extravehicular activity (EVA) spacesuit presents an even more challenging scenario for managing CO 2 levels near or below 2 mmHg. Furthermore, one could argue that astronauts performing an EVA are more likely to be called upon to make difficult decisions during their work outside the core habitat while in an EVA spacesuit.…”