Coal
Authors: Attfield Michael , Castranova Vincent , Kuempel Eileen , Wagner Gregory 
Published: August 2005

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. Research on the health effects of coal has been heavily concentrated on the more common anthracite and bituminous types. There, the results have shown more of a gradient in toxicological effect across coal types rather than any clear‐cut divisions between types. Little research has been undertaken on lignite and other brown coals. This has prohibited drawing firm conclusions on its toxicity relative to its harder cousins. This lack of any obvious demarcation has led us to report on overall patterns of effect rather than to provide separate presentations by coal type. Where data are available, we draw attention to any findings especially relevant to any particular coal type.

Coal varies considerably in composition and consists largely of carbon, hydrogen, and oxygen with smaller amounts of sulfur, nitrogen, trace elements, and metals. Coal originated from mostly organic material that was long ago buried by sediments. Heat and pressure converted the plant remains over geologic time to coal, the process thought to have led to peat, lignite, bituminous coal, and anthracite in turn. Coal, as extracted, contains many minerals in various proportions, including quartz, clays, carbonates, and sulfides. These minerals can be intrinsic to the coal, as in silica grains within the coal matrix, or may lie in pockets or layers. Although there are different methods for classifying coal—each developed for a specific geological or economic purpose—all tend to relate to the age of the coal. Older coals are purer, harder, more brittle, have less volatile matter, and have higher calorific value. They also contain higher amounts of fixed carbon. Fixed carbon is the basis of a commonly used classification index of coal—coal rank. High rank coals, such as anthracite, have the greatest amount of fixed carbon, whereas bituminous and subbituminous coals are defined as low rank. A rank‐based system devised by the American Society for Testing and Materials (ASTM) divides coal into four main subtypes: anthracite, bituminous, subbituminous, and lignite. Each subtype is further divided, using fixed carbon, moist Btu, or other factors. For example, the main anthracite group is divided into meta‐anthracite (98% or more fixed carbon), anthracite (92–98% fixed carbon), and semianthracite (80–92% fixed carbon). The lignite group is split into lignite and brown coal; the main distinction is whether the substance is consolidated (lignite) or not consolidated (brown coal).

Exposure to “pure” coal, that is, the dust generated from processing washed and cleaned coal, is rare. Most occupational exposures are to coal mine dust, an entity with varying properties, that contains coal and also dust generated from the accidental or deliberate cutting of the rock strata adjacent to or within the coal seam. The rock strata can be formed of limestone, shale, clay, or sandstone and therefore can give rise to silica and other dusts, which mix with the coal dust to give coal mine dust exposures. Consequently, coal mine dust contains 30–40% of noncoal minerals, of which about 5–10% is typically silica. The major inorganic constituents of coal ash. Virtually all of the findings on health outcomes in humans are derived from studies of exposures to coal mine dust.