> The point was that PM2.5 is a measurement of particle size, and that by itself allows no judgement about its toxicity.

This does not logically follow at all. The size indicates where it can reach in the lungs, whether cilia can eject it, etc.

A 5cm ball shot at the head at high speed is indeed dangerous. We are talking about inhalation of particles causing irritation, and the size is indeed the major factor. Content as well, but frying pan particles filled with carbon chains that have gone through who knows what reactions are indeed of concern. Lots of extremely nasty things are easily accessible from chains of hydrocarbons, from toluene to formaldehyde.

> The toxicity judgement comes from the information what substance has the form of PM2.5, and the journo managed to omit that.

I believe the journalist is not at fault here in the least. The scientific papers I have seen usually class all PM2.5 together, and perhaps by source. But the size itself is of great concern due to the size allowing easy entry to the body that is not possible for larger sizes.