An air purifier filter system comprises an air purifier filter unit and a machine readable identifier which provides information concerning the filter unit type and performance information specific to a target pollutant to be filtered by said air purifier filter unit from a plurality of different pollutants that can be filtered by that filter unit type. This information can be used to provide an end of life prediction for the filter unit or provide advice that the filter should be changed for other reasons. This enables the user to be given advance warning, and to be given instructions when it is time to change the filter unit. The instructions to change the filter unit may be more accurate than a simple timer as they can take account of the type of filter unit and the performance information. In some examples the use history of the filter unit may also be used to predict the time when it should be changed.