Abstract: Amongst other things, an air filter apparatus is provided having a filter element and a frame defining a first axis and having a plurality of rails connected to one another for supporting the filter element in removably surrounded relation between the rails. The filter element is convertible between an extended configuration and a collapsed configuration, and spans the length of said first axis in the extended configuration. The filter element is adapted to releasably engage two or more of the rails, thereby to selectively retain the filter element in the extended configuration thereof, and so as to place the air filter apparatus in an operative configuration thereof.