Landfills are the third largest human-made source of methane in the world, after livestock and natural gas.,  Landfill gas is about half methane and half carbon dioxide (CO2), laced with hundreds of toxic contaminants, including methylmercury and many chlorinated chemicals that can form highly toxic dioxins when burned (see Waste Incineration). Radioactive tritium is also increasingly found in landfill gas, from emergency exit signs and other sources. Methane is a greenhouse gas, 86 times more potent than CO2 over a 20-year period. It is produced when organic discards (food scraps, paper and wood products, yard waste, sewage sludge) decompose in an oxygen-starved environment.
In the United States, larger landfills are required to capture landfill gas (and usually burn it), but capture systems are only partially effective. Landfills claim that they typically capture about 75% of their gas,,  but reality can be much lower.,  Much of the gas escapes as fugitive emissions, causing cancers and other health problems in neighboring communities.
Many landfills burn gas in a flare, while others burn it to make heat or electricity, or clean it up to feed into natural gas pipelines. These are called Landfill Gas-To-Energy (LFGTE). Burning the gas converts most of the methane to CO2, drastically reducing the global warming impacts. While breaking down some pollutants in the gas, it also produces new ones like nitrogen oxides that trigger asthma attacks and ultra-toxic dioxins. LFGTE projects receive many state and federal subsidies. Climate and energy policy-makers are lobbied by the waste industry to subsidize landfills and incinerators instead of supporting zero waste solutions like composting and recycling. As a result, some communities even cancel composting programs to dump more organics in landfills to maximize LFGTE opportunities.,  Nearly 90% of materials discarded in landfills and incinerators can be recycled or composted. Typically competing with wind and solar in renewable energy mandates, landfills also absorb subsidies that ought to flow to these cleaner non-burn alternatives.
Ironically, burning landfill gas for energy can be worse than simply burning off the gas. Managing landfills as energy facilities encourages the mismanagement of landfills to make them more gassy, when a proper waste system would do the opposite. Food scraps, yard waste and other clean organic materials should be separated at the source and aerobically composted. The dirty remaining organic materials in the trash and sewage sludge should be anaerobically digested to stabilize it before landfilling to avoid methane generation in landfills, where gas is harder to capture.
Energy Justice Network: energyjustice.net/lfg
Global Alliance of Wastepickers: globalrec.org