【Compatibility】The 511 replacement filter is only compatible with Blue Pure 511.Fit snugly with the machine 【Value Pack】Including 3 packs 511 2-in-1 replacement filters at low cost to save your money.（Note:The fabric pre-filter is not included in the package） 【Premium Filter】Only using premium filter to efficiently absorb most of airborne particles in your home or office 【Odor Filter】The inner activated carbon filter helps to absorb unwanted odor such as pet odor,cooking odor,smoke and VOC 【Replacement Frequency】We recommend to replace the filter every 6-12 months depending on using environment and frequency Filter