For several types of granular solids (sands, silicon carbide and copper shot), penetrations have been measured for a monodisperse aerosol (I. I mu m Dow microspheres) passing through fly ash deposited upon horizontal surfaces of the solids. Fractional penetrations can run as low as 10(-5). Pressure drop data for dust cake/granular medium combinations are also given. The data illustrate significance of dust autohesivitiy and dust/granular solid adhesivity for granular-bed filtration assisted by dust cake formation. (c) 2005 Elsevier B.V. All rights reserved.