This makes me wonder, for a truly travel-size solution (or just otherwise compact needs), if there's a viable alternative that directs filtered air at a sleeper's head? In other words, would the net effect be equivalent for one person (such that they're breathing more or less the same cleanness of air)?

I grasp that such an approach might draw unfiltered surrounding air along with the filtered stream of air, and imagine that it would likely be easily overwhelmed by a steady supply of exceptionally polluted air (wildfire smoke or simply a heavily polluted urban area), but I still would be interested in knowing if the compromise never overcomes a level of placebo effect, or if there's a point where it actually achieves measurable benefit (both in efficacy and portability—like a even more compact and directional version of the Corsi-Rosenthal computer fan solution.)