The process is also affected by the permeability of the ground and other factors, such as pressure differences in the soil, cavities, pipes, and tunnels.
These gases can include methane (CH4), carbon dioxide (CO2), hydrogen (H2), and volatile organic compounds (there are approximately 500 others that can be present in trace forms) from the waste on site and its degradation over time.
Steps must be taken to prevent this migration from the landfill site as it might enter buildings in the vicinity.
This can be done on the site by means of combinations of geomembranes and clay based products, see gas protection.
Guidance for this in the UK can be found in CIRIA C665 [2] and also BS 8485 and Title 40 of the United States Code of Federal Regulations, parts 239 through 282.