aerodynamic diameter

Aerodynamic diameter is a clever way of describing an inhaled particle not by its true physical size or shape, but by how it behaves in moving air. Formally, it is the diameter of an imaginary smooth sphere of unit density (1 g/cm³) that would fall through air at the same speed as the real particle. It answers the practical question: regardless of what this particle looks like, where in the lung will the airflow carry it?

This matters because deposition in the airways is governed by aerodynamics, not geometry. A dense particle or an irregular fluffy one can have very different physical and aerodynamic sizes; what determines whether it crashes into the throat, settles in the bronchi, or drifts into the alveoli is its aerodynamic diameter. As a rough guide, particles above ~5 µm tend to impact in the mouth and throat, those around 1–5 µm reach the conducting airways and deep lung, and those below ~1 µm are largely exhaled again.

For an inhaled product, the spread of aerodynamic sizes is usually summarized by the mass median aerodynamic diameter (MMAD) — the size below and above which half the aerosol mass lies. Formulators measure it with cascade impactors, which sort particles into stages by their inertia. Engineering the MMAD into the right window (and using low-density porous particles to make a large light particle behave aerodynamically small) is central to getting drug to the intended lung region.