In this paper the performance of a thin-walled sampling tube operating in calm air, pointing both vertically upwards and downwards, is investigated. The problem is considered mathematically and the sampler is modelled as a long circular cylinder. Two mathematical models have been developed to determine the flow field around the sampler, one model assumes potential flow and the other takes into account the viscous effects of the flow. From detailed knowledge of the airflow, the paths of suspended particles in the fluid flow were calculated and the aspiration efficiency of the sampler determined. The dependence of the efficiency upon the particle size and suction rate are considered for various operating conditions and a comparison made with available experimental data. The results obtained using the two different flow models are compared and hence the effects of assuming potential flow, an assumption often made when studying aerosol sampling, are investigated. It is found that neglecting the viscous effects of the fluid results in a higher predicted value of the aspiration efficiency, A. However, in all the cases considered, the difference in the values of A predicted is never large and the general behaviour of A is always the same. (C) 2002 Elsevier Science Ltd. All rights reserved.