It is impossible to tell which ones in the whole tangle of the properties of atmospheric aerosol particles (ambient particulate matter, PM) are crucial to the PM environmental and health hazardousness. It seems that there is not much reason in attributing the PM adverse effects to some selected, “main”, intrinsic PM properties, because they easily lose or acquire the importance depending on time and place, the PM effects being site and time dependent. Furthermore, this attribution may be erroneous because the observable health or environmental effects are usually not due to PM only; they rather derive from the concordant actions of PM and accompanying circumstances, the latter in general unknown.