Comment by mindslight

I'd say basically all of them in this target market - a respirator with check valves is much more comfortable to use.

For the beginning of Covid I modded my 7500 (half face) to remove the check valve flaps and seal up what would normally be the exhaust ports, making both inhale and exhale go through the 2297 filters. It was harder to breathe, more humid, and developed liquid water accumulation over longer periods.

I would think that if you did the same thing with a full face respirator, it would immediately fog up. If you look at the flow path on them, it's something like atmosphere -> filter -> top face portion -> mouth nose portion -> atmosphere, presumably to keep the part you need to see out of full of dry air.