Figure 1: A Marple-type cascade impactor. Each compartment of the impactor (stages 1 to 8) and the slit size (mean value) of each stage. The slit size is indicated by width and length (stages 1 to 6) and diameter (stages 7 to 8). In a cascade impactor, the flow velocity through the slits increases in more distal stages. The situation is the complete opposite in the human airway.