Do you use HEPA filtration?

We don’t use HEPA filters because they don’t make the oxygen cleaner.  An oxygen concentrator is a thousand times more selective than a HEPA filter.

An oxygen concentrator needs to separate oxygen at 299 picometers from nitrogen at 302 picometers.   A HEPA filter can only filter particles over 300,000 picometers.   So HEPA filtration does not affect oxygen quality.