Methane (CH4), CH4
Methane is a colourless gas, odourless at low concentrations, but with a sweetish chloroform-like odour at high concentration. It is highly combustible, and mixtures of about 5 to 15 percent in air are explosive. Upon release into the atmosphere methane is destroyed by reactions with other chemicals in the atmosphere, giving a lifetime of about 10 years.
As the major constituent of natural gas, methane is burned to directly heat homes and other commercial buildings. It is also used as a fuel in power stations to produce electricity. Methane is used widely in the chemical industry in the production of more complex chemical compounds.
Methane occurs naturally in the environment. One of the major sources is from the decomposition of plant and animal matter by methane producing bacteria. These occur in air-less environments such as marshes and the gut of some animals and landfills. Methane is also trapped in pockets within the earth's crust, and can be released during the mining of fossil fuels. On a global scale, the human activities that result in the most methane emission, in descending order of importance are: livestock farming, production of fossil fuels, wet rice cultivation, biomass burning, landfill and domestic sewage.
Excessive exposure to methane may affect the brain and ultimately leads to suffocation. Methane gas build-up from landfill sites is a potential explosion hazard. In the past this has resulted in a few temporary evacuations of residents in housing estates built on top of old landfill sites that have not sufficiently vented the methane. As a volatile organic compound (VOC) it has been determined to have negligible photochemical reactivity and is unlikely to contribute significantly to the formation of harmful ground level ozone or photochemical smog.
The main environmental concern with methane is the role it plays as a greenhouse gas influencing climate change. The concept of global warming potential has been developed to compare the ability of each greenhouse gas to trap heat in the atmosphere relative to another gas. Carbon dioxide has a global warming potential (GWP) of 1 (over 100 years). Although less methane is emitted into the environment the global warming potential of methane is 21 times that of carbon dioxide (over 100 years).