The small size of the methane molecule explains several advantages of natural gas as a resource and as a fuel. As a gas, it is highly compressible and has a very low viscosity; these properties make it more efficient to produce than oil (a typical gas reservoir may give up 60 to 90% of the "gas in place", vs 20-30% for oil.) When gas is accidentally released to the atmosphere, it is of little environmental consequence since it is lighter than air and usually dissipates readily. As a fuel, it has few contaminants that are common in oil and coal, so it burns extremely cleanly. On combustion, it breaks down into water vapor and carbon dioxide, releasing a third less CO2 per BTU than oil, and half that of coal.