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 at-mosphere on human health and issues of removing it from the atmosphere under conditions of pressurized modules of space stations. It is shown 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, what occurs probably when СО2 content in the atmosphere is ≈7.6 mm Hg (1.0 %). These nominal values, which arose at the origin of life, when CO2 content in the Earth's atmosphere was orders of magnitude greater than now, can be observed in all living things that breathe with lungs. Whenever CO2 concentrations in human air vesicles and arterial blood go below these values, the human experiences oxygen deprivation, even when there is a high oxygen concentration in the blood. The use of СО2 for breathing in medicine, aviation, and athletics also does not support the opinion that СО2 content in atmosphere below 7.6 mm Hg affects human health. Data from NASA studies of the link between СО2 levels in the ISS’s atmosphere below 6.0 mm Hg (0.8 %) and astronauts’ headaches does not prove this link. NASA’s unfounded lowering of standards for СО2 content down to 2.0–3.0 mm Hg requires an increase in the number of systems for СО2 scrubbing onboard the space station. At present, according to the minutes of AQS toxicological subpanel of the international ММОР panel, no scientifically proven standards currently exist for СО2 content in the atmosphere of space vehicles; their substantiation requires further scientific studies conducted by testers and cosmonauts.