There is a lot of variation in humans. I believe that some people can easily handle CO2 while others get panic attacks. That study, of CO2 effects on submarin

There is a lot of variation in humans. I believe that some people can easily handle CO2 while others get panic attacks. That study, of CO2 effects on submariners, suffers from selection bias. Here's a US Navy presentation from 2003 that indicates that 10% of submariners quit during their first assignment for psychological reasons: http://annex.ipacweb.org/library/conf/03/bing.pdf http://annex.ipacweb.org/library/conf/03/bing.pdf I believe many of those people quit because they experience anxiety or other negative effects from moderate CO2 levels. Many others never apply to the submarine service in the first place because they have experienced CO2-triggered anxiety which they believe is claustrophobia. Some anecdata: https://www.reddit.com/r/newtothenavy/comments/439j6f/for_prospective_submariners_how_do_they_test_for/ https://www.reddit.com/r/newtothenavy/comments/439j6f/for_pr... Apparently prospective astronauts must pass a CO2 tolerance test: https://www.forbes.com/sites/quora/2016/02/05/could-you-pass-the-quirky-claustrophobia-test-nasa-gives-to-astronauts/#2e0c15dd26b4 https://www.forbes.com/sites/quora/2016/02/05/could-you-pass... EDIT: Submariners get several CO2 tolerance exams, as reported on Quora: > At Basic Enlisted SubSchool (BESS) you are tasked with quite a few things that test you for claustrophobia. The old Ascent tank (to practice submarine emergency escapes I understand that is is no longer used) had air locks where you would crowd and entire class into the chamber (in Navy terms 'nut to butt'), and pressurize it so you could open the hatch into the tank and then free ascent (in a Stankehood) to the surface. Lots of fun, washed out a few who couldn't deal with the air lock. > Later in the training they piled us all into a decompression tank and took us to atmospheric pressure equaling some depth or other (it's been a while... almost 40 years) and then they keep an eye on you. > In theory, it's to see if you can handle the air pressure... during the long wait (more than three hours,) due to having the Midnight to 6 AM watch in one of the admin buildings the night before I dozed off. The instructors woke me up when they cracked the hatch and let people out. I thought I was in trouble, but they laughed at me and told me that it was really a 'final exam' in detecting claustrophobics... and my falling asleep proved I didn't have it. https://www.quora.com/How-claustrophobic-is-it-to-be-in-a-nuclear-submarine https://www.quora.com/How-claustrophobic-is-it-to-be-in-a-nu...