This paper presents experimental data for penetration of nanoparticles in the 4-30 nm size range through fibreglass filters perforated with defined pinholes at 5 and 15 cm/s. Results show that, when the filter is damaged. penetration increases with decreasing particle diameter, This lower performance is more significant for filters with high airflow resistance. A model explaining the experimental data has been designed based on balanced, laminar airflow inside the pinhole; these conditions were confirmed by detailed study. The experimental points agreed very closely with this model. (C) 2009 Elsevier Ltd. All rights reserved.