This is Starwaster's branch of Yongedevil's Ioncross Crew Support, a life support mod.
With Ioncross Crew Support installed, Kerbals must now make allowances for oxygen consumption and carbon dioxide buildup. Long duration missions should plan on having regenerators to convert CO2 back into Oxygen. Failure to plan accordingly may result in loss of life and mission failure.
Changes in this version from the original are primarily to address bugs in CO2 scrubbers. Future updates will be aimed at greater and lesser difficulty configurations (ex. CO2 scrubbers requiring cartridges) and ensuring that dead Kerbals stay dead.