Re-circulating hoods draw in steam and odour saturated air through a filter, then returning the treated air into the kitchen. If ducting for extraction is not possible, then recirculation may be another option. In this case, the air is drawn through a charcoal filter that removes cooking odours and steam, and the clean air is recirculated back into the kitchen. Cooker hoods for recirculation are easy to install and are normally used in blocks of flats where installing ducts leading directly to the outside of the building can be difficult. Extraction hoods draw in cooking fumes and trap vapours in a filter while expelling odours to the outside through a ventilation duct. You can expect to have to change the charcoal filter once or twice a year, depending on how often the cooker hood is used.