Abstract: An air filter element incudes a plurality of filter paper units each composed of a sheet of filter paper corrugated into a number of folds extending from one side to the other of the sheet, the filter paper units being arranged such that ridges of the folds extend on or parallel to radial lines extending radially outwardly from a center of the air cleaner element, and at least one connector interconnecting side edges of the filter paper units. With the air cleaner element incorporated in an air cleaner, air drawn into the air cleaner from an air inlet flows into a space above the air cleaner element, and then flows through the air cleaner element at a uniform speed over the entire area of the air cleaner element. Therefore, the air can uniformly be filtered, and the resistance to the air flow of the air cleaner element is reduced. The area of the air cleaner element which is open to the air flow is several times greater than the cross-sectional area of the air inlet.

Abstract: A filter medium for filtration of a fluid to flow therethrough from an upstream side to a downstream side thereof, said filter medium comprising a substrate having a density gradient wherein the density is high on the upstream side and low on the downstream side of the substrate.