What PM2.5 Actually Measures

PM2.5 is the mass of particles with an aerodynamic diameter of 2.5 micrometres or less, suspended in one cubic metre of air, reported in micrograms per cubic metre. It is a mass figure, so a small number of larger particles within that band can outweigh a great many smaller ones. Two rooms reporting the same value can therefore contain very different particle populations.

The 2.5 micrometre boundary describes a sampling efficiency curve rather than a hard cut. Instruments collect a declining fraction of particles as size increases through the boundary region, which is why two monitors sampling the same air can disagree while both remain within specification.