How does a exhaust fan work?

Cooking produces more than aromas. Particles of grease, smoke and combusted gasses are environmental pollutants that are not healthy to breathe in. Some foods like pepper and onion also produce irritants which can be unpleasant to be continuously exposed to. Exhaust fans are used to remove these pollutants from the air. There are two types of exhaust fans: venting and recirculating. Venting exhaust fans use a powerful fan to pull air in from the kitchen and out through a duct which generally empties to the outside of the house. Generally, the duct is as short as possible to allow the fan to work more efficiently. Recirculating exhaust fans are actually air filters. The fan sucks air from the range hood through a filter, and blows it back out in the kitchen. The filter removes grease from the air. Recirculating fans do not improve air quality as much as venting fans, but they are easy to install and require little space. A venting exhaust fan needs to vent properly. Sometimes, venting fans are routed to the attic instead of outside. The constant flow of warm, moist air can cause rot, mold, and other health problems. Using too long a duct can also be a problem. The longer the duct, the more the fan has to work and the less air it can push through the system. Finally, there is the issue of replacement air. If the fan is pulling air out of the house, more air has to come in to replace it. In old buildings that aren't very airtight, this isn't much of an issue. In new buildings, however, people will often install a grill to the outside where new air can come in to replace the air pulled out.