A hydrocarbon with chemical formula C₆H₆, benzene contains six carbon atoms in a ring structure. A clear volatile liquid with a strong odor, it is one of the most extensively used hydrocarbons. Because it is an excellent solvent and a necessary component of many industrial chemicals, including gasoline, benzene is classified by United States federal agencies as a known human carcinogen based on studies that show an increased incidence of nonlymphocytic leukemia from occupational exposure and increased incidence of neoplasia in rats and mice exposed by inhalation and gavage. Because of these cancer-causing properties, benzene has been listed as a hazardous air pollutant under Section 112 of the Clean Air Act.