There is nothing inherently impossible about the idea that all airborne substances of some specific size are harmful to breathe. It simply requires that they be bad because they physically fit into somewhere that shouldn't have foreign substances of any kind in it rather than because of something specific to the substance.
To be just a touch pedantic, that all particles of a certain size are harmful is a silly assertion.
Water and sugar particles, for instance, are almost certainly harmless below some reasonable threshold.