Methane is an odourless, colourless gas. It is the main component of natural gas. Methane is highly combustible in the range from 5 % to 95 % volume in air, hence a certain caution is essential when dealing with methane. There are two main areas where methane has to be measured. One is as a product of inefficient combustion. Methane can be present in the flue gas from certain types of burner, especially when unusual or mixed fuels are burned. The other main application is in biogas systems, where methane is produced as a result of anaerobic digestion of waste. Usually this is planned decomposition, but older landfill sites will also produce methane, where it is then treated as an unwanted waste product instead of a valuable resource. Good quality biogas consists of around 80 % methane, which is easily enough for most combustion purposes. Methane is the lightest of the alkanes, chemically CH4. Due to its small molecular size methane is quite difficult to measure, the most common method being infrared.