Masonry and prefabricated fireplaces can be fueled by wood, natural gas, biomass and propane fuel sources. Ventless Fireplaces (duct free/room-venting fireplaces) are fueled by either gel, liquid propane, bottled gas or natural gas. In the United States, some states and local counties have laws restricting these types of fireplaces. They must be properly sized to the area to be heated. There are also air quality control issues due to the amount of moisture they release into the room air, and oxygen sensor and carbon monoxide sensors are safety essentials. Direct vent fireplaces are fueled by either liquid propane or natural gas. They are completely sealed from the area that is heated, and vent all exhaust gasses to the exterior of the structure.