When considering how to change your air purifier filter, its worth remembering that you have a two-part filter in your Dyson machine. Dyson Purifier Hot Cool Formaldehyde purifying fans have two sealed filters - a replaceable glass HEPA carbon filter, and a permanent catalytic filter. In normal use, we would expect the HEPA carbon filters to last for 12 months if used for 12 hours a day, or 6 months if in constant use. The catalytic filter is permanent and does not need to be maintained or repla