Caught an interesting study out of the University of Leicester. It seems that some particulates in air pollution impact the behavior of bacteria in one's respiratory tract: how they grow and form communities, and thus how they are able to combat our immune system.
While the study is concerned with carbon particles, common in highly polluted big city exterior air, it does seem to me to have some potential bearing on what we ought be concerned about in indoor air. Some studies have toyed with the idea that it is as much the existence of heavy particle load, rather than specifically what particles are involved, that impacts (generally irritates) the respiratory tract.
That's why, in emphasizing health benefits in providing commercial janitorial service, we do our best to keep client indoor air as particle free as possible, using HEPA filtered vacuums (on tile as well as carpet), microfiber dust wipes rather than feather dusters, microfiber damp mops, and so on.
Doing what we can, in an uncertain world.