Abstract: The present invention relates to a filter medium comprising a substrate and a fine fiber layer on top of the substrate, wherein the substrate comprises a first layer comprising first fibers having a first average diameter and a first maximum fiber length; a second layer comprising second fibers having a second average diameter and a second maximum fiber length; and a third layer comprising third fibers having a third average diameter and a third maximum fiber length; wherein the boundary area between the first and the second layer forms a first blended area comprising first and second fibers; and the boundary area between the second and the third layer forms a second blended area comprising second and third fibers; and wherein the first and the third average diameters are each larger than the second average diameter.