ISO 22252:2020

This document specifies performance requirements and gives recommendations for the design of breathing air supply and CO2 absorption systems of manned submersibles, capable of maintaining suitable life support conditions in the manned compartments.
It is applicable to manned submersibles where the internal pressure of the manned compartment is normally maintained at or near to one atmosphere.
It is not applicable to submersibles where the occupants endure pressures higher than one atmosphere (such as in diving bells, for example).
It is not applicable to submersibles designed to carry passengers or divers in a separate compartment capable of being pressurised to higher than one atmosphere inside the pressure hull (such as in submarine rescue compartments, for example).