Mines can be classified as coal, metal, and nonmetal. Coal, antimony, lead, tungsten, molybdenum, and silver are mined primarily underground. Diesel exhaust is an important hazard of underground mining. Carbon monoxide and nitrogen dioxide are hazardous gases encountered in blasting. Exposure to silica dust is a major concern for hard rock metal miners, but not for coal miners. [Burgess, p. 423-34] See "Pneumoconiosis among underground bituminous coal miners in the United States: is silicosis becoming more frequent?" [PMID 19773275] See the agent "Hematite mining, underground, with radon exposure."