Expandable pleated air filters

A pleated air filter comprising a pleated layer of nonwoven electret filter media and a pleated stiffening layer attached to the layer of nonwoven electret filter media and co-pleated with the layer of nonwoven electret filter media, wherein the pleated air filter is expandable, along an expansion direction, from a first, compacted condition to a second, in-use condition, and wherein the pleated air filter comprises:

• first and second opposing primary ends that are non-corrugated ends and that respectively comprise first and second securing tabs;
• and,
• first and second opposing secondary edges that are corrugated edges,
• wherein the nonwoven electret filter media is compliant and wherein first and second opposing corrugated ends of the nonwoven electret filter media respectively extend outward beyond first and second opposing corrugated edges of the stiffening layer, in a direction at least generally aligned with a pleat direction of the pleated air filter and for a distance of 3 mm to 25 mm, so that first and second opposing corrugated edges of the nonwoven electret filter media provide the first and second opposing secondary corrugated edges of the pleated air filter and are self-sealing when abutted against inward secondary sidewalls of a filter frame in which the pleated air filter is installed.