It is widely accepted that the aerodynamic diameter of a particle is one of the main factors that determines particle deposition into the human respiratory system. The determination of aerodynamic diameter of spheres or near spherical objects is routinely accomplished using impactors. The aerodynamic diameter of man made organic fibers (MMOF), on the other hand, has not traditionally been measured using impactors, because fibers of the same cross section may have different lengths and a variety of shapes (straight, curved, etc.) for each length. The aerodynamic size of the fibers is thus a function of fiber orientation. Single and multiple stage impactors have been developed, calibrated, and validated specifically for the determination of the aerodynamic diameter of large fibers with circumscribed diameters between 20 and 35 μm and an aspect ratio ranging from subfiber lengths (aspect ratio <3) up to 40. The impactor allows measurements of the aerodynamic diameter of cellulose acetate fibers released during mechanical smoking of cigarettes. The performance characteristics were evaluated by spherical particles of known diameters, fibers of known length and diameter, and computational fluid dynamic calculations. Our methodology has shown that inertial impactors can be used to determine the aerodynamic diameter of large cellulose acetate fibers.