COAL

Coal is an organic, combustible, rock-like natural substance that occurs in various forms from hard and brittle anthracite to soft and friable lignite. Coal is sometimes classified into two types: hard coal and soft coal. These terms do not, however, have a standardized meaning. One definition calls anthracite hard coal and places all other coal types in the soft coal category. A more common convention is that of Speight, in which anthracite and bituminous coals are termed hard coal and lignite and brown coal are classified as soft coal.

Coal exists as distinct seams of mineral laid down as sedimentary deposits within the earth. The seams are extracted by surface (strip) or underground mining. Surface mining generally involves the removal of rock and other strata to uncover the top or side of a coal seam. Using blasting and/or mechanical means, the coal is fractured and removed from the mine by train or truck. There are two main types of underground mining: room and pillar and longwall (each accounting for 50% of U.S. production in 2010). Room and pillar mining is the traditional method of mining, whereby pillars of coal are left to support the roof. These pillars may later be removed or “robbed,” a process that provides better reclamation of the coal but speeds up subsidence of the overlying strata. Room and pillar operations are commonly undertaken using conventional mining or continuous mining. In the former method, the bottom of the coal seam is undercut first, followed by blasting to bring the coal down. In continuous mining, a machine with a rotating cutting head removes the coal from the coal face. Longwall mining, of more recent origin, involves removing “slices” of coal from the edge of the seam by large coal cutting machines. All of the coal is extracted, and the roof is left to collapse as the coal face advances. A lesser-used method of underground mining is the shortwall system, which is a hybrid of the longwall and continuous mining methods and is used principally in Australia. After extraction, coal is typically crushed and cleaned. Cleaning can involve screening for size, removal of impurities by wet or dry methods, and drying.

A black mineral that consists mainly of carbon, used as a fuel and as a source of organic chemicals. It is the fossilized remains of plants that grew in the Carboniferous and Permian periods and were buried and subjected to high pressures underground. There are various types of coal, classified according to their increasing carbon content.

There is little epidemiologic evidence of a relationship between coal mine dust exposure and lung cancer development, although a link with silica dust exposure has been reported. Histological evaluation of lung tumors in coal miners reveals that these tumors vary little in cell type or pathological features from those associated with cigarette smoking.
Elevated stomach cancer rates have been reported in studies of U.S. underground coal miners compared to general population rates. A mechanistic explanation for coal dust-induced stomach cancer suggests that swallowed coal dust mixes with nitrates in food and under the acidic conditions in the stomach, nitrosation of organic material associated with this coal dust occurs, resulting in the production of carcinogenic products. Evidence indicates that nitrosation of coal dust extracts become mutagenic and can cause neoplastic transformations in mammalian cells. Some evidence of elevated stomach cancer mortality in coal miners has also been observed in more recent studies in the United Kingdom, Germany, and United States.