I think it's worth mentioning to people that for non-infection purposes, HEPA is honestly not a great idea.

The problem is that such low-porosity filters struggle to even let air move through them. And if you don't move a lot of air, you aren't cleaning a lot of air. At low fan speeds they're effectively placebo for how little they do.

You can get around this by adding yet more powerful fans and cranking up their speed, but then it sounds like a jet engine. Everyone just ends up turning them off (or way down) because they're too noisy - defeating the purpose.

If you just want non-virus stuff (pollen, smoke, dust) then don't go HEPA. Go with something like MERV-13. They come in standard off the shelf furnace style filters. They're way cheaper than HEPA and way better for most use cases.

HEPA only makes sense for something as fine as viruses, or if you only get a single pass to do the filtering. For anything that circulates (like in a house) it makes 0 sense.