A gas filter which is formed from a single continuous sheet (11) of material folded along a plurality of parallel fold lines and interconnected at appropriate folds so as to produce an element having spaced front and rear faces (42,43) and a plurality of partition walls (44,45) extending between said front and rear faces and defining therebetween a plurality of chambers (40,41) of wedge-shaped cross-section, the wedge-shapes of adjacent chambers being oppositely orientated, and, said sheet being formed with apertures such that gas can flow through the filter from the front face (42) to the rear face (43) and in so doing is constrained to pass through apertures in at least one partition wall so as to pass through at least two adjacent chambers.