Recirculation is a way of operating a fume cupboard in which the intake air is not ducted out of the building, but is returned filtered back into the room. In this mode, the cooker hood draws in the vapours and odours produced during cooking and passes them through two types of filters. First, the air passes through a grease filter, which traps grease, and then continues through a special carbon filter. This filter contains activated carbon, which is designed to absorb and neutralise odour particles. This cleaned air, free of grease and odour, is then blown back into the kitchen. However, unlike extraction to the outside, recirculation does not remove moisture and heat from the room. Carbon filters need to be changed regularly as they become clogged and lose their absorption capacity over time. This mode is the solution for kitchens where it is not possible to provide an exhaust duct to the outside.