massamedianediameter

Mass median aerodynamic diameter (MMAD) is a standard metric used in aerosol science to describe the size distribution of airborne particles. It is defined as the aerodynamic diameter at which 50% of the total aerosol mass consists of particles with smaller or equal aerodynamic diameter. The aerodynamic diameter is a notional size that reflects how a particle behaves in air, incorporating size, density, and shape so that particles with the same aerodynamic diameter deposit similarly in the respiratory tract or in deposition chambers.

MMAD is typically determined by collecting an aerosol with a cascade impactor or similar instrument and measuring the mass of particles collected on each stage. The data are often fit to a lognormal distribution, and the MMAD corresponds to the 50th percentile of the mass distribution. The number-based median diameter may differ from the MMAD, because MMAD weights larger particles more heavily by mass.

Interpretation and use: MMAD informs where inhaled particles are likely to deposit in the respiratory system. Smaller MMAD values (roughly 1–3 μm) favor deeper lung deposition, while larger MMAD values deposit mainly in the upper airways or are exhaled. Typical MMAD values vary by application, with pharmaceutical inhalers aiming for MMADs around 1–5 μm, depending on target deposition.

Related concepts include AMAD (activity median aerodynamic diameter) used in radiological contexts and the geometric standard deviation (GSD), which describes distribution spread. Limitations include reliance on density assumptions and sampling methods, meaning MMAD is a summary descriptor rather than a complete distribution.