A phenomenological model is presented to explain the increase in pressure drop (Delta p) of air filters during steady operation with oil mist. It is based on (currently) semi-quantitative conclusions obtained from measurements of liquid distribution patterns in the media associated with the transport of coalesced liquid by the flowing air. Correlation of these patterns in space and time with the evolution of the pressure drop suggests that the over-all increase in Delta p (the "wet" pressure drop) is governed by two distinctly different liquid transport mechanisms: A steep Delta p jump is required to overcome the capillary exit (or entry) pressure and pump liquid into non-wettable, or out of wettable fibrous matrices. It is associated with the formation of a thin liquid film covering almost the entire front (or rear) face of the respective media. With the help of a polymerization technique to "freeze" the liquid distribution, the film is shown to be confined to the outermost surface without entering the media while the aerosol flow is on. Liquid transport inside the media is shown to occur in multiple parallel channels spanning almost the entire thickness of a filter. The channel Delta p associated with this transport mechanism increases linearly with media thickness. Wettable media form numerous fine channels which feed a liquid film on the rear face by which drainage takes place. Non-wettable media form fewer, relatively wide channels ending in large drops on the rear face, through which drainage takes place during steady operation. Sandwiched combinations of wettable and non-wettable media show the same combination of features in their respective Delta p curves. There are separate Delta p jumps and channel regions for each media type. In case of a transition from wettable to non-wettable media, the combined exit and entry Lip jump takes place at the internal interface. (C) 2013 Elsevier Ltd. All rights reserved.