Median aerodynamic diameter (MAD) is one of two parameters influencing the deposition of inhaled particles, the other being the geometric standard deviation of the particle size distribution. The MAD is the value of aerodynamic diameter for which 50% of some quantity in a given aerosol is associated with particles smaller than the MAD, and 50% of the quantity is associated with particles larger than the MAD. It simplifies the true distribution of aerodynamic diameters of a given aerosol as a single value. It is also used to describe those particle sizes for which deposition depends chiefly on inertial impaction and sedimentation.