[kkfx]

The point is probably about concentration: if certain "pathogens" (no matter their nature, it's valid from all bacteria, viruses, poisonous chemicals etc) are present but below a certain concentration they are effectively harmless. If more concentrated they might be harmful. Under certain condition a local concentration might form a colony that act as a persistent source of potentially harmful "pathogens". A bit more complex, but a good generic description.

What it's not clear to me is the definition of "purifier" and they operating principle: VMCs are normally harmless because they do not blow humid air and they keep air flowing so it's hard to offer room for "colonization" in most cases. That's the same principle we have for modern water heaters, classic ones, large pressurized balloons filled of hot sanitary water do require a regular daily extra-heat-up against legionella, modern ones with a not pressurized hot-water balloons and just a spiral pipe inside to "instantaneously" heat the sanitary water flowing in the pipe in most country do not demand anti-legionella heating cycle because they do not offer room for it's development. So much of the point of this study should be define the kind/family of devices they have testes.