This study investigates particle collection efficiency of porous foam within an impaction cup by experimental and numerical techniques to clarify why the cut-off aerodynamic diameter (Dpa(50)) of porous substrates tends to decrease as thickness and porosity increase. Careful measurements confirm this tendency in the particle size ranging from 1 to 10 mu m and with foams of varying porosity. The measurements serve as the basis for a numerical simulation of particle trajectories, which demonstrates that particles can penetrate into the foam to varying degrees, dependent also on the Reynolds number and their distance from the jet axis. The proposed numerical models are then used to investigate the influence of substrate thickness and porosity on particle collection efficiency of porous foam within an impaction cup. The simulations explain and quantify the ratio in Dpa(50) as a function of foam properties, compared to a solid impaction surface. They also show that the steepness of efficiency curves declines slightly with foam thickness and porosity, compared to a solid substrate and at the same Reynolds number. (C) 2010 Elsevier Ltd. All rights reserved.