An air/gas filter medium is comprised of a substantially homogeneous core layer of feather down clusters held together by a binder to form a 3-dimensional interconnected filament structure throughout the core. The filament structure forms air pocket and connect and trap the feather down clusters which form minute air pockets between its filaments and hairs in which undesirable substances from an air/gas flow therethrough are captured. An air permeable scrim sheet is retained on a surface of the down cluster core. The down cluster core is compressed to a predetermined density to modify the average size of said minute air pockets to trap known undesirable substances from the air/gas flow.