Interest in the performance of aerosol samplers operating in slow moving, or calm, air has increased in recent years as windspeeds in many current workplaces are only of the order 10-1 m/s. In such situations one of the main mechanisms by which particles are brought to the vicinity of the sampler is gravitational settling. This is an important difference from sampling in fast moving air where gravitational effects are negligible. Hence much of the understanding that we have gained into the performance of samplers operating in moving air cannot be applied in calm, or slow moving, air situations. In this work the study undertaken by Dunnett et al (2006) is extended to consider general expressions for the aspiration efficiency of a sampler operating under various conditions in calm air.