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. The presence of elevated concentrations of dihydrogen (H2) in the landfill gas is also consistent with elevated temperatures at remote locations some distance from the gas-extraction well. The presence of H2 is consistent with thermal inactivation of -reducing microbes, which normally combine all H2 produced by fermentation of organic acids with to form methane (CH4). H2-producing microbes are less temperature-sensitive than -reducing microbes so that elevated temperatures can inactivate them and their recovery can be delayed over the H2-producers. This can result in H2 production without the (usually) corresponding consumption, resulting in elevated concentrations of H2 in the landfill gas (up to >25%[v:v] at some sites). Thermal deactivation of CO2-reducing microbes has been used to produce CO2 (rather than methane) from municipal solid waste (Yu, et al., 2002).