i have a neighbor (or three) that smokes weed and another that burns incense. the combo sometimes infiltrates my apartment (through the closet, where i think there is a hole). combined with the old building smell that's embedded in the framing, it's an awful mix of air pollutants. one of my air purifiers does double duty between the closet and the kitchen to combat this very problem.

with that said, activated charcoal (which my purifiers include, though undersized) is more suited to smells (molecular size) than hepa filters (particulate matter, which is generally much larger) are. hepa in the bathroom would deal with airborne bacteria but not the poo smell, which would require activated charcoal (a lot of it, along with an uncomfortably powerful fan) to be really effective.