The influence of CO2 in a space-like environment: study design.

For the operation of manned spacecraft, the removal of CO2 from the cabin atmosphere, produced by its inhabitants, is essential. This is accomplished by chemical absorption in a gas processing unit, a process which requires energy and consumables. Therefore, in terms of resource management, the CO2-level should be kept as high as possible. Otherwise, considering crew health and performance and also the interference with life science experiments, the CO2 load should be as on Earth, close to zero. In order to obtain more information about the permissible CO2 level for future space missions and also to clarify Space Station design criteria, NASA-ESA-DARA have initiated a ground-based simulation study with two different CO2 levels: 0.7% (first campaign) and 1.2% CO2 (second campaign). For this study the deep diving facility of DLR was used to provide atmospheric control and long-term habitation for the test subjects in studying the effect of increased CO2 on physiological and psychological functions. A number of experiments were implemented, which tested selected effects of raised CO2 on humans. Four male subjects stayed in the chamber for 26 d in each campaign, in order to perform the different tests in repeated trials, with the aim of evaluating possible long-term effects. CO2 was controlled by absorption with soda lime, flushing with fresh air and the addition of CO2, if necessary. Essentially, the CO2 produced by the subjects was used to maintain the level at 0.7 and 1.2%, respectively. Basic control of CO2 was carried out in the soda lime container of the Life Support System. In order to maintain the required level, the amount of gas flowing through the soda lime could be adjusted by a remote controlled bypass. With this set-up it was possible to keep CO2 at an average level between 0.67 and 0.73% in the first campaign and between 1.17 and 1.23% in the second campaign. The results of the experiments support the current CO2 limits for space operations, insofar as values around 1% do not impose any severe restrictions to human habitation for at least several weeks, whereas life sciences experiments especially sensitive to CO2 influences have to be carefully evaluated for possible interferences.