Abstract: An air filter apparatus includes at least one frame that is configured to receive a filter media, and at least one sensing mechanism for sensing contamination of the filter media. The sensing mechanism can include a wave emitter and a wave detector. The wave emitter and the wave detector can be arranged so that the wave travels along a path that is substantially parallel to outer faces of the filter media. The wave detector can be located at a first edge of the frame, and the wave emitter can be located spaced apart laterally from the wave detector at a second edge of the frame, so that waves from the emitter can travel along the path through and laterally across the filter media towards the detector. The path can be substantially perpendicular to the general direction of air flow within the filter media.