Maintaining allowable concentration of carbon dioxide in the atmosphere of habitable pressurized modules of space stations

The paper discusses the effect of carbon dioxide (СО2 ) concentration in the atmosphere on human health, and issues relative to necessary CO2 scrubbing in the space station pressurized compartment environment. It shows that, according to respiratory physiology, СО2 is a gas essential for human respiration, which turns into a toxic contaminant, when its concentration exceeds the threshold of 40 mm Hg (5,3%) in human lung alveoli, and, accordingly, of 46–49 mm Hg (6,0–6,5%) in arterial blood, which apparently occurs when СО2 content in the atmosphere exceeds 7,6 mm Hg (1,0%). Not enough reasonable NASA decision to reduce normative levels of СО2 partial pressure on the ISS since 2008 from 5,3 mm Hg due to headache of several astronauts requires an increase of a number of СО2 removal systems. In the meantime, the investigation of NASA specialists indicated a relationship between headaches in astronauts and CO2 level increase up to 7,6 mm Hg (1,0%) and more in the inspirable air in sleep stations during sleep. Long-standing investigations on the ISS during long-duration flights coordinated by ESA, NASA and JAXA Committees have shown no relationship between astronauts headache and СО2 concentration in the ISS pressurized compartments atmosphere containing 2,7 ± 0,5 mm Hg СО2.