Very small particles with aerodynamic particle diameters of less than 0.1μm are primarily collected by Brownian diffusion because they have little inertial impaction (bumping) due to their small mass. Interception is limited by their reduced physical size. This bumping causes them to move first one way and then another randomly (diffused) through the gas. The Brownian diffusion process leading to particle capture is most often described by a dimensionless parameter called the Peclet number,Pe(USEPA-81/10, p. 9-8):