Landfills produce a mix of gases, but the two dominant ones are methane and carbon dioxide. By volume, landfill gas is typically 45% to 60% methane and 40% to 60% carbon dioxide. The remaining fraction includes nitrogen, oxygen, ammonia, sulfides, hydrogen, and small amounts of toxic organic compounds.
The Two Main Gases
Methane is the gas most associated with landfills, and for good reason. It’s flammable, potent as a greenhouse gas, and produced in enormous quantities as buried waste breaks down without oxygen. Carbon dioxide makes up nearly as much of the gas by volume. Together, these two gases account for roughly 90% to 98% of all landfill emissions.
Both gases form through the same basic process: bacteria consume organic material (food scraps, paper, yard waste, wood) and release methane and carbon dioxide as byproducts. This is the same type of decomposition that happens in swamps and the guts of cattle, just concentrated underground in massive quantities.
Trace Gases and Why They Matter
The remaining sliver of landfill gas, typically less than 10% by volume, contains a cocktail of other substances. Nitrogen makes up 2% to 5%, leftover from air trapped in the waste. Oxygen is present at very low levels, between 0.1% and 1%. Ammonia and sulfides each account for roughly 0.1% to 1%.
The most concerning trace components are non-methane organic compounds, which make up just 0.01% to 0.6% of the gas but include chemicals like benzene, vinyl chloride, and trichloroethylene. These are volatile industrial compounds that leach from household products, paints, cleaners, and other items buried in the waste. At the concentrations found in landfill gas they rarely pose a direct health threat to nearby communities, but they’re the reason landfill gas collection systems exist in the first place.
Hydrogen sulfide deserves special mention because it’s responsible for the rotten-egg smell people associate with landfills. The human nose can detect it at extraordinarily low concentrations, as little as 0.008 parts per million. That means you can smell a landfill long before the gas reaches levels that would cause any physical irritation, which typically starts around 6 to 10 ppm.
How Landfill Gas Forms Over Time
Gas production doesn’t start immediately or stay constant. It follows four distinct phases as bacteria work through the buried waste.
In the first phase, oxygen is still present in the freshly buried refuse. Aerobic bacteria (the kind that need oxygen) break down organic material and produce carbon dioxide, water, and heat. This phase is short, lasting only days to weeks until the trapped oxygen runs out.
Once oxygen is depleted, the second phase begins. Anaerobic bacteria take over and start producing organic acids as they break down cellulose and other plant-based materials. Carbon dioxide and hydrogen are the main gases during this stage, but methane production hasn’t started yet.
The third phase is where methane production ramps up. A different group of bacteria begins converting those organic acids into methane and carbon dioxide. Gas output accelerates and eventually reaches its peak. The fourth phase is a long, slow decline as the easily digestible organic material gets used up. Methane production continues but at a decreasing rate, and this tail end of gas generation can last for decades.
Why Methane Is a Climate Concern
Methane from landfills is a significant contributor to climate change. U.S. landfills account for roughly 17% of the country’s total human-caused methane emissions. Globally, the figure is close to 20%.
The reason methane gets so much attention relative to carbon dioxide is its heat-trapping ability. Over a 100-year period, a ton of methane from non-fossil sources (which includes landfills) warms the atmosphere about 27 times more than a ton of carbon dioxide, according to the IPCC’s most recent assessment. That makes landfill methane one of the more impactful greenhouse gases by source, especially because it’s generated continuously from waste that was buried years or even decades ago.
Explosion and Fire Risk
Methane is odorless and colorless on its own, which makes it difficult to detect without instruments. It becomes explosive when it reaches a concentration between about 5% and 15% in air. Below 5%, there isn’t enough fuel to ignite. Above 15%, there isn’t enough oxygen. That narrow window is exactly the range landfill gas can hit if it migrates through soil into enclosed spaces like basements, utility tunnels, or buildings near a landfill.
This is the primary safety hazard of landfill gas. The methane itself isn’t toxic at the concentrations typically found near landfills, but an undetected buildup in a confined space can cause an explosion. Landfill operators monitor for methane migration at the site boundary specifically to prevent gas from reaching that 5% threshold in nearby structures.
How Landfill Gas Is Captured
Modern landfills use gas collection systems to prevent uncontrolled emissions. These systems consist of networks of perforated pipes, installed either vertically as wells or horizontally as trenches throughout the waste mass. Vertical wells typically extend to depths of 50% to 90% of the total waste thickness.
In passive systems, gas rises naturally through the wells due to pressure differences. Active systems add vacuum pumps to pull gas out more aggressively. Either way, the collected gas is routed through pipes to a central processing point.
The simplest disposal method is flaring, where the gas is burned off in open or enclosed flare stations. This converts methane into carbon dioxide, which is still a greenhouse gas but far less potent. More advanced setups use the gas as fuel for turbines, boilers, or internal combustion engines that generate electricity. Some facilities clean the gas to pipeline quality and inject it directly into natural gas distribution networks.