The Purair® ductless fume hoods are a series of high efficiency products designed to protect the user and the environment from hazardous vapours generated on the work surface. At the heart of the Purair fume hood product line is the innovative Air Science Multiplex™ Filtration Technology that creates a safe work environment over the widest range of applications in the industry. The Premium Ductless Fume Hoods use the 11 lbs fume hood filters. The 24 inch and the 36 inch Fume Hoods use one filter each and the 48 inch Fume Hood uses two filters. In order to pro-long the life of the main filter you can purchase the optional pre-filter which will pick up any particles in the air to prevent blockage of the main filter.