"NASA put forward in 2008 requirements to reduce the level of carbon dioxide in response to subjective complaints from some astronauts about headaches at agreed CO2 levels of 5-6 millimeters of mercury column. At the same time, no medical examination of the crew was carried out to determine the possible causes of the headache", the report said, as cited in the training centre's scientific magazine.
Though the level of CO2 has since been gradually reduced as requested by the US side, astronauts continue to complain of headaches, with the current concentration at less than 3 millimeters of mercury, the report said.
In 2014, NASA released an article based on a detailed study of the link between CO2 levels in the ISS atmosphere and astronauts' headaches. When only Russia's CO2 removal system operated at the station, as the US' one was turned off, the CO2 concentration increased up to 6.2 millimeters of mercury column and astronauts were feeling irritability and fatigue, NASA said.
Russian specialists have refuted the link, saying that Russian and foreign cosmonauts flying to the ISS over the past 30 years had not complained of headaches, with an average CO2 level in the atmosphere of the stations of about 6 millimeters of mercury. Among possible causes of such headaches, they listed the impact of weightlessness, associated with a rush of blood to the head, as well as the fact that astronauts breathed in the exhaled air with increased content of CO2.