A fibrous filter is a common cleaning device often used to remove particles from industrial gas streams. The main question which often arises concerns the evolution of the pressure drop and the filtration efficiency during the filter clogging. In the present study, the loading characteristics of HEPA filters have been studied experimentally. The increase of pressure drop and filter efficiency was measured and was linked to both the penetration profile inside the filter bed and the deposit structure observed thanks to scanning electron micrograph. We have also studied the influence of various parameters such as air velocity, par-tide size, aerosol concentration and filter main characteristics. A depth and surface filtration model has been developed based on the distinction between the fibres of the filter and deposited particles resulting in additional fibres inside the filter or on the filter surface. We can notice a good agreement between model and experiment. Moreover, model very well describes the transition area between depth filtration and cake filtration. This transition from one type of filtration to another is a continuous process. Model describes also the exponential decrease of penetration profile.