METHANE/NATURAL GAS (MARSH GAS)
A common hazard in many manholes is the presence of marsh gas— also known as natural gas, coal damp, and swamp gas. Natural gas is a mixture of several gases and the end product of anaerobic (in the absence of oxygen) decomposition of organic material. It is found in concentrations or deposits by itself, in conjunction with petroleum, and in deposits near coal. Its presence in manholes usually is the result of decomposition of organic material in the sewer.
Methane—a flammable, nontoxic, colorless, odorless, and tasteless gas—constitutes from 70 to 95 percent of natural gas, depending on the location of the deposit. Although natural gas and methane have some technically important differences, they are very similar in the properties pertinent to emergency responders. Although methane is nontoxic, the organic material in the manhole may break down into some gases, such as hydrogen sulfide, that are toxic.
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