Measurements of aerosol penetration through porous polyurethane foam plugs were carried out in order to develop a model describing the aerosol penetration behaviour. The data were found agree well with an existing semi-empirical model for porous foam aerosol penetration. This model was improved by reformulating it to use consistent units, and accurately measurable input parameters. The model was implemented as part of a design spreadsheet, to design foam modules suitable for a range of specific aerosol monitoring applications. The design spreadsheet was validated by comparing its outputs with the known characteristics of a number of pre-existing foam modules, It predicted the characteristics of these modules accurately, within the limitations due to a lack of knowledge of the precise cell diameters of the foams used to make them. The use of the design spreadsheet to explore different options for size-selective foam modules is illustrated. The results obtained highlight the importance of a reliable supply of foam, having closely specified cell diameters, in order to manufacture size-selective samplers meeting acceptable quality standards. Crown Copyright (C) 2001 Published by Elsevier Science Ltd. All rights reserved.