Protec Continuous Cleaning Cartridge, 1 PackLove this product! Never have to worry about the sludge forming in the water container of humidifier or the smell of humidifier's water! (smells 'funky' if it has bacteria) For myself and my partner, it has cut down on any pseudomonas bacterial sinus colds since placing this product in the humidifier's water chamber. Recommend you buy in bulk, so you have it at the ready when next winter comes along.