particle size

Particle size is how big a single grain of powder is. For a perfect sphere this is simply its diameter, like asking how wide a marble is. The trouble is that drug particles are almost never spheres: they are flakes, needles, jagged crystals, or clumps, so saying how big they are is less obvious than it sounds.

Because of this, scientists use the idea of an equivalent diameter: the diameter of a sphere that behaves the same way as the irregular particle by some chosen property. An equivalent volume diameter is the diameter of a sphere with the same volume; an equivalent surface diameter matches the surface area. Different measurement methods report different equivalent diameters, so two instruments can honestly give different numbers for the same powder.

Particle size matters because it controls so much downstream behaviour. Smaller particles have more surface area per gram and therefore dissolve faster, which can raise the absorption of poorly soluble drugs. But very fine powders flow badly, stick together, and can be hazardous to inhale, so smaller is not automatically better.

A practical caveat: a quoted particle size is meaningless without saying how it was measured. A size from sieving, from laser diffraction, and from microscopy are not interchangeable, and reputable specifications always name the method.