An article, comprising: a first phase comprising a first plurality of fibers and a second plurality of fibers different from the first plurality of fibers; a second phase comprising a third plurality of fibers and a fourth plurality of fibers different from the third plurality of fibers, the third plurality of fibers being the same as or different from the first or second plurality of fibers and the fourth plurality of fibers being the same as or different from the first or second plurality of fibers; and a transition phase between the first and second phases, wherein the transition phase comprises at least a portion of the first plurality of fibers, at least a portion of the second plurality of fibers, at least a portion of the third plurality of fibers, and at least a portion of the fourth plurality of fibers, and wherein at least a portion of the first plurality of fibers, at least a portion of the second plurality of fibers, at least a portion of the third plurality of fibers, and at least a portion of the fourth plurality of fibers are intermingled with each other, wherein the air permeability of the first plurality of fibers is higher than the air permeability of the second plurality of fibers, and the air permeability of the third plurality of fibers is higher than the air permeability of the fourth plurality of fibers, wherein the air permeability is determined using a handsheet produced exclusively by such fibers and having a basis weight of 100 g/m 2 according to the standard ISO 9237, wherein the first and third pluralities of fibers are made from an organic polymeric material, wherein a weight ratio of the first and second pluralities of fibers is from about 50:50 to about 90:10, and a weight ratio of the third and fourth pluralities of fibers is from about 10:90 to about 50:50,wherein the basis weight ratio of the first phase to the second phase is from 30:70 to 70:30, and wherein the first and second phases form a wet laid structure that is configured as a filter medium. a first phase comprising a first plurality of fibers and a second plurality of fibers different from the first plurality of fibers; a second phase comprising a third plurality of fibers and a fourth plurality of fibers different from the third plurality of fibers, the third plurality of fibers being the same as or different from the first or second plurality of fibers and the fourth plurality of fibers being the same as or different from the first or second plurality of fibers; and a transition phase between the first and second phases, wherein the transition phase comprises at least a portion of the first plurality of fibers, at least a portion of the second plurality of fibers, at least a portion of the third plurality of fibers, and at least a portion of the fourth plurality of fibers, and wherein at least a portion of the first plurality of fibers, at least a portion of the second plurality of fibers, at least a portion of the third plurality of fibers, and at least a portion of the fourth plurality of fibers are intermingled with each other, wherein the air permeability of the first plurality of fibers is higher than the air permeability of the second plurality of fibers, and the air permeability of the third plurality of fibers is higher than the air permeability of the fourth plurality of fibers, wherein the air permeability is determined using a handsheet produced exclusively by such fibers and having a basis weight of 100 g/m 2 according to the standard ISO 9237, wherein the first and third pluralities of fibers are made from an organic polymeric material, wherein a weight ratio of the first and second pluralities of fibers is from about 50:50 to about 90:10, and a weight ratio of the third and fourth pluralities of fibers is from about 10:90 to about 50:50,wherein the basis weight ratio of the first phase to the second phase is from 30:70 to 70:30, and wherein the first and second phases form a wet laid structure that is configured as a filter medium.