 Monitoring of the landfill gas itself can be used diagnostically.When there is concern regarding the possibility of an ongoing subsurface oxidation event, or landfill fire, the presence in the landfill gas of compounds that are more stable at the high temperatures of such an event (above 500 °C) can be evidence for such a process occurring.The presence of propene, which can be formed from propane at temperatures above several hundred degrees Celsius, supports high temperatures.