Uhm, I know (or think I know) what your talking about. I've always seen them advertised as low power consuming air filter device things. The purpose of it is to do as FZ said, but some actually use hepa filter looking things to catch the dust and pollen and such.
I imagine that if you lived in a dusty or high pollen count environment, such a device could provide some happiness, so long as it truly cleans the air.
But I don't think the negative ions have anything to do with triggering happines. If this were the case, it seems the whole world would be incredibly happy. If nothing else, I would point out to her that negative ions are all over the place, tv, probably natural occurances or big power stations, and that the world, for the most part, is miserable.