The core structural difference
Alkaline electrolysers move ions through a liquid alkaline solution between two electrodes, usually with a porous diaphragm rather than a gas-tight barrier. PEM electrolysers move protons through a solid polymer membrane that also acts as a physical separator.
Everything else in this comparison follows from that one structural fact.
Gas separation
Because PEM keeps the half-reactions on opposite faces of a membrane, hydrogen can be collected alone and the oxygen vented. In a typical consumer alkaline or “HHO” design, both gases come out of the same outlet in roughly a 2:1 ratio by volume.
That ratio is why mixed-gas flow figures inflate so easily: about a third of the quoted volume is oxygen. A 1500 mL/min mixed-gas claim is not 1500 mL/min of hydrogen.
Feed water and consumables
Alkaline systems require the electrolyte to be prepared, topped up and eventually disposed of. It is caustic, and handling it is a real ownership task.
PEM systems take distilled water and nothing else. The trade is strictness: feed water quality directly determines stack longevity, so “just use tap water” is not an option.
Cost, response and durability
Alkaline technology is older and cheaper per unit of output at industrial scale, which is why it persists there. PEM stacks cost more, respond faster to load changes and package into much smaller, quieter units — exactly the constraints a consumer appliance has.
Neither technology is universally better; they optimise for different problems. For a device someone breathes from at home, separation and clean feed water dominate the decision.
How to read a spec sheet
Ask three questions of any inhaler listing: is the cell PEM/SPE or alkaline, is the quoted flow separated hydrogen or mixed gas, and has purity been measured by an independent laboratory.
If any of the three is unanswered, the headline number cannot be compared with anything.