incombustible gas

Typically this uses nitrogen, carbon dioxide or argon, which are incombustible gases and so inhibit combustion.

Mud volcanoes are often associated with lava volcanoes, and the typical relationship is that where they are close, the mud volcanoes emit incombustible gases, while the ones further away emit methane.

They are also often associated with lava volcanoes; in the case of such close proximity, mud volcanoes emit incombustible gases including helium, whereas lone mud volcanoes are more likely to emit methane.

Majoor and O'Brien explain that the Hindenburg was designed to be inflated with helium, an inert, incombustible gas, but that the American government, which controlled the world's largest supply, would not sell it to the Germans.