The modelling of the variation in the pressure drop across a HEPA filter during cake filtration in the presence of humidity was studied. A semi-empirical model was developed that considers the pressure drop across the clogged filter as the sum of the pressure drops across the clean filter and across the cake, which is itself decomposed into several successive layers. The pressure drop across each layer of particles depends on its state of equilibrium with the air humidity. The model includes the specific resistance of each layer of particles and its variation when the cake is exposed to a flow of moist air. This model applies for relative humidity below the deliquescent point of the aerosol if it is hygroscopic. The results show good agreement between the calculated values and experimental values for flat filters clogged with aluminium oxide and sodium chloride test aerosols. The model also revealed the influence of filtration time on the evolution of the pressure drop across a clogged filter in the presence of humidity, especially with a hygroscopic aerosol. (C) 2010 Elsevier B.V. All rights reserved.