Originally Posted by
Flapping_Madly
But: is it not the case that this freshly filtered air may be clean of germs etc but is still a mixture of fresh and bleed air? So the theory won't hold up.
I hope you can understand that. Not one of my best. Thanks.
Most modern aircraft ECS systems use a ~50/50 split of 'fresh' air from the engine bleeds and 'recycled' air. That air is mixed before it gets to your overhead vent, so in that sense it's all the same. The recycled air is run through a HEPA filter, which is extremely effective at filtering out germs/viruses and similar particulate contaminates - but would be ineffective for gaseous contaminates. Your technique of trying to breath only air from the overhead vent would help prevent you from catching a cold from the person sitting next you. But if the engine bleed air is somehow contaminated it wouldn't help significantly.