Masonry and prefabricated fireplaces can be fueled with 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 US, some states and local counties have laws restricting these types of fireplaces. They must be sized appropriately 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.