That’s true. If you assume it’s a lot more than just the O2 supply, it makes more sense. Ok. I have a new fun headcanon.

So, here’s the thing. We know the Federation has multiple species in it. Those species likely have different atmospheric requirements. Sure, we sometimes see crew members wearing breathing apparatus, like the Benzites. But I wonder if every species actually needs its own atmospheric mix? Maybe every species has its own ideal mixture of oxygen, nitrogen, CO2, etc.

That is a problem if everyone has to share the same atmosphere. But what if they didn’t have to? Imagine if the life support system is generating little individual micro atmospheric pockets for every person onboard. Every crew member basically gets to breathe the atmospheric mix of their home planet. Using transporters, force fields, replicators, and good-old-fashioned fans, the ship generates little atmospheric bubbles around each crew member. And those bubbles move with them through the ship.

So would anyone need a breathing apparatus? It’s possible those species just have very unique air requirements that go beyond the capabilities of the life support system to handle. For example, maybe the Benzites require a gas that is poisonous to some other species? You don’t want to make big pockets of that floating around. You minimize the amount of it introduced to the ship by using personal breathing devices.

This would also explain why the system is capable of dispersing arbitrary compounds.

But the need for constant filtration is an interesting point as well. Though that seems like less of an immediately lethal threat than a long-term exposure thing.