Definition
Purity is a statement about the gas leaving the machine: what fraction is hydrogen, and what else is present. The categories that matter are residual oxygen, water vapour carry-over and any material shed from the cell stack or gas path.
A purity figure is only meaningful with a method attached — who measured it, with what instrument, and against which reference.
Modality relevance
Purity matters most for inhalation, where the gas is breathed directly, and remains relevant for bathing because the same cell feeds the diffuser.
Purity relevance
Solid polymer membrane design is the reason PEM output can be characterised at all: hydrogen evolves on one side of the membrane and oxygen on the other, so the streams are physically separated rather than mixed and later filtered.
Science relevance
Purity verification belongs to metrology, not clinical science. A verified purity number says the hardware performs as specified; it makes no statement about outcomes.