Coal the heat and pressure of deep burial causes the loss of water, methane and carbon dioxide and an increase in the proportion of carbon. Thus first lignite (also called "brown coal"), then sub-bituminous coal, bituminous coal, and lastly anthracite (also called "hard coal" or "black coal") may be formed. The wide, shallow seas of the Carboniferous Period provided ideal conditions for coal formation, although coal is known from most geological periods. The exception is the coal gap in the Permian–Triassic extinction event, where coal is rare. Coal is known from Precambrian strata, which predate land plants—this coal is presumed to
Health effects of coal ash environment. When coal ash waste is not properly controlled, the toxic substances can affect drinking water, food and air. Coal ash contains many toxic substances that can negatively impact the human body. Employees working in coal-fired power plants or near coal ash waste sites are at major risk of inhaling coal ash dust. Coal ash dust is generally known as particulate matter (particle pollution) and the dust particles can harm the lungs when inhaled. Workers increase their risk of harmful side effects when they inhale the smallest coal ash dust particles. The smaller the coal ash dust particle, the deeper