a frame carries a variable flow scent dispenser and supports an air filter . the frame , dispenser and filter are to be inserted as a unit into a forced air heating and / or cooling system duct adjacent to the duct outlet through a floor or wall register . the filter is in the form of a two - ply fibrous sheet with an opening between the plies to receive the frame and dispenser . the filter collects dirt particles borne by the warm air as it leaves the duct and is quickly and easily replaceable . the scent dispenser is a container for scented air freshening material and has apertures through opposite walls , which walls are disposed transversely to the direction of airflow , permitting warm airflow through the container . the warm air will cause evaporation from the scented material and will entrain the vaporous scent and disperse the scent throughout the room . a slide is adjustable to increase or decrease the aggregate area of the apertures in one of the apertured container walls so that the intensity of the air freshening scent carried into the room can be controlled .