At least by the series 12 version the suit had an AI that was programmed to deactivate the organic components — i.e. the wearer. Nardole recognised the interface as his ex, Velma which could be deactivated so long as the suit wasn't damaged.
They were programmed so provide air in terms of credits, the more credits one had the more breath they had. The default set of breaths appeared to range between 2500 to 3000. As Nardole confirmed, the more one breathed, if panicking for instance, the quicker they'd die. (Oxygen)