The collection efficiency of a fiber filter of polyester operating in the removal of nano-sized aerosols was studied here. The polyester filter utilized had 0.882 in porosity, 4.5 turn in thickness, and fiber mean diameter of 16.0 mu m. A TSI Electrospray Aerosol Generator, model 3480, was used to generate monodispersed nanometric particles, with sizes varying from 8.5 to 94.8 nm. The nanoparticles were generated from solutions of NaCl which were atomized as droplets, followed by drying. The filtration device was built in acrylic with a filtration area of 0.4 m(2) . The filter efficiencies were obtained by counting the particles before and after the filter, using a TSI Condensation Particle Counter, model 3007. The tests were accomplished at gas superficial velocities varying from 0.03 to 0.25 m/s. The results show the efficiency decreasing with increasing particle size, indicating the predominance of the diffusional mechanism in the range studied. Also, the filter efficiency decreased with filtration velocity. The results were compared to well known theoretical predictions based on the classical collection mechanisms: diffusional and direct interception. The comparison of the calculated collector efficiency with the experimental results showed that the predictions underestimate the results. A correction was proposed to the existing correlation for direct interception, where an 'effective' diameter accounting for the Brownian motion of the particle while in the vicinity of the collector surface was accounted for. The proposed modification included no fitting parameter. The results showed a considerable improvement in the prediction correlation. (c) 2007 Elsevier B.V. All rights reserved.