Originally Posted by
GoFast8
Im no mechanic but Im pretty confident that the air filtration in a typical airliner is basically nonexistent.
For a PAX, air comes into the cabin from above and is exhausted below you at foot-level.
That air is then recirculated through a HEPA filter, mixed with bleed air via the backs, and sent back to you. Repeat.
Some cabin air is exhausted overboard via outflows or small leaks around door seals, etc. That's why there's continuous inflow from the packs.
So the air you're getting is either bleed air (outside air further sanitized by engine compressor temps) or cabin air recirced through a HEPA filter. Your seat-mate could still cough in your face, but you won't be getting anything from the HVAC.
The pack outlet might or might not have a different filter to adsorb any fumes from the engine/APU.