Methane – the unseen hazard, is colourless, odourless, explosive when mixed with air in specific proportions and is also an asphxiant. It is a common constituent of gases that evolve in "landfill sites" and is often wrongly assumed to be the "landfill gas." However, landfill gas is a complex cocktail of constituents that include amongst others methane, carbon dioxide, oxygen, hydrogen sulphide and trace organic vapours. Like methane, carbon dioxide is also toxic but not flammable, and together they form the principal components of landfill gas.

Methane is produced by the biodegradation of organic matter under reducing conditions and there are natural as well as man made sources for both this and carbon dioxide.

The volumes and levels of hazardous gases produced by landfill sites vary considerably during their life cycle and are controlled by a wide range of factors. Generally, landfill containing biodegradable materials is characterised by the initial production of large volumes of carbon dioxide and a relatively small proportion of methane. However, within a short period of time, methane volumes can increase significantly and carbon dioxide reduces correspondingly. The biodegradation process can continue for periods in excess of thirty to forty years, but, the volumes and concentrations reduce with time.

Building Regulations require that "precautions shall be taken to avoid danger to health and safety caused by substances found on or in the ground to be covered by a building." If either methane or carbon dioxide enter buildings or structures they can pose a risk to health and safety. Both are classified as contaminants according to Building Regulations.

For there to be a hazard there has to be a source producing the gas(es) and a pathway via which migration can occur to a target or receptor. Methane has a lower density than air and is buoyant but carbon dioxide is in contrast heavier than air. Therefore their relative proportions in any ground gas will affect its properties and in particular mobility. However, regardless of density, gas will migrate from a high to low pressure area.

Where there is a risk of a gas hazard, be it from a landfill site or natural source, an assessment of the presence of methane and carbon will be required. A F Howland Associates have considerable experience in hazardous gas assessment for new development and employ a range of investigation techniques. Working throughout the UK on we are able to offer the following services: -

Desk studies and appraisals Ground investigations In-situ gas monitoring

Hazardous gas assessment and remediation advice Expert advice and litigation studies