A kitchen exhaust fan, also called an exhaust hood, is a mechanical fan that sits above the cooktop in the kitchen. The fan's purpose is to remove grease that has become airborne, combustion products, smoke, odor and other fumes through the evacuation of the air combined with proper filtration.
In most exhaust fans or hoods, a special filtration system is put into place that has a grease trap to remove and store grease along with other particles. In many large kitchens, the exhaust hoods are used along with special fire suppression devices, so if a grease fire does occur, the fumes are properly vented out so the fire does not spread and can be taken care of quickly.
Commercial kitchen vent hoods are also often combined with a fan that brings in fresh outside air to combine with the cooking fumes, and then all of the fumes are blown out through the hood, making the process safer. Although most kitchen exhaust fans release air and other fumes outside, some simply recirculate the air back into the kitchen. These recirculating systems typically use filters and other methods to ensure that grease and cooking odors do not get back into the air.