By Siobhan Molloy, SST Sensing & Rachael Barton, GSS

Within a self-contained system where humans are respiring, there will be a build up toxic CO 2 levels over time. As a consequence, in any form of spacecraft where humans are present, effective mechanisms for removing CO 2 and resupplying oxygen need to be implemented. However, operational necessities still dictate that these will only be efficient enough to keep the CO 2 levels down to a certain extent.

As air moves differently in such locations, without the influence of gravity involved. Heat does not rise and this in turn leads to significantly less mixing of air taking place. The occupants onboard the International Space Station (ISS) are thus exposed to greater levels of CO 2 during their time onboard than the rest of us experience. Down here on Earth it represents just 0.03% of the air’s content (equating to a partial pressure of 0.23 mmHg). NASA has set a long-duration spacecraft maximum allowable CO 2 concentration that is more than double that figure, at 0.7% (a partial pressure of 5.3mmHg).

Though extensive terrestrial studies have shown that this increased level will have no effect on the continued good health of those exposed to it, there must be some acknowledgement of the fact that crew members onboard the ISS still regularly experience symptoms of acute CO 2 toxicity – such as headaches or lethargy.

It is generally accepted that human adaptations to microgravity conditions will cause astronauts in space to become more sensitive to elevated CO 2 levels. This can result not only in physical discomfort, but also impinge on their cognitive skills and reaction times – thereby potentially leading to safety risks. On average crew members stay on the ISS for a period of around 6 months, so having a good grasp of the ongoing implications is clearly of great value.

With operational practicalities meaning that it is not possible (either technically or financially speaking) to remove enough CO 2 to replicate normal conditions here on Earth, the ISS has to function with relatively high ambient CO 2 concentrations present in the air. Yet its occupants have to live and work in an acceptable environment.