Aerodynamic diameter is a physical property of a particle in a viscousfluid such as air. In general, particles have irregular shapes with actual geometric diameters that are difficult to measure. Aerodynamic diameter is an expression of a particle's aerodynamic behavior as if it were a perfect sphere with unit-density and diameter equal to the aerodynamic diameter. Such a model has the same terminal settling velocity.
Aerodynamic diameter is commonly applied to particulate pollutants and inhaled drugs to predict where in the respiratory tract such particles will deposit.
Drug particles for pulmonary delivery are typically characterized by aerodynamic diameter rather than geometric diameter. The velocity at which the drug settles is proportional to the aerodynamic diameter, d<sub>a</sub>.