Most of the commonly used bioaerosol samplers are single-stage impactors that meet the conventional Marple's design criteria: their non-dimensional jet-to-plate distance, S/W, is greater than the established threshold (1.5 for rectangular nozzles and I for the circular ones). Recent studies have shown that these samplers underestimate the concentration of airborne fungal spores because their cut-off size is about 2.5 mu m (Air-O-Cell and Burkard samplers) or greater while some fungal species produce spores of ca. 1.8-2.5 mu m in aerodynamic diameter. In this study, we evaluated the single-stage circular-jet impactors with very small jet-to-plate distances (S/W << 1). The laboratory and field data obtained with test particles of different sizes and different origin (biological and nonbiological) demonstrated the feasibility of these "incorrectly designed" impactors for the spore collection and total enumeration (viable + non-viable spores). A decrease in the jet-to-plate distance resulted in a critical decrease of the impactor's cut-off size (d(50)): from 2.5 [mu m to about 1 mu m. This reduction of cut-off size makes such an impactor efficient for collecting spores of all fungal species (>= 1.8 mu m) and even some bacterial species (>= 1 mu m). Since the spore surface density across the circular deposit area was non-uniform, three sample reading procedures were evaluated: the entire area count, random partial count, and a partial count on a rectangular "diametric slice". The collection efficiency data suggested that a relatively small jet-to-plate distance is likely to result in excessive shear forces in the impaction zone, thus enhancing the spore deaggregation and bounce. The coefficient of inter-sample variation of the field samples, collected by commercially available impactors with S/W approximate to 0.099, did not exceed 50% for the total spore count. The highest variability was observed for Arthrospores, which were more aggregated than other types of fungi. (c) 2004 Elsevier Ltd. All rights reserved.