Although the use of monodisperse aerosols to measure penetration through sampling devices results in accurate measurements, it is time consuming and expensive. Measurement methods using polydisperse aerosols are orders of magnitude faster, but tend to be restricted to smaller particle diameters. This is particularly the case with systems using the Aerodynamic Particle Sizer (APS) in calm air, which are limited by poorly characterised aspiration efficiency and low detection efficiency above approximately 10 mu m. By measuring the aspiration of such a system in calm air, and introducing a virtual impactor to act as an aerosol concentrator above 10 mu m, we have developed an APS-based penetration measurement system that may be used successfully up to 20 mu m. Experimental errors (+/- 1 S.D.) are typically less than 20-25% at 20 mu m, reducing to below an estimated 10% at 15 mu m. Errors below 10 mu m are of the order of a few percent. The system is ideally suited to the rapid and inexpensive development of size selective aerosol samplers with cut points below 15 mu m, and is capable of quantifying the performance of such samplers under calm air conditions. Crown