Mixtures of dispersed combustible materials (such as gaseous or vaporised fuels, and some dusts) and oxygen in the air will burn only if the fuel concentration lies within well-defined lower and upper bounds determined experimentally, referred to as flammability limits or explosive limits.

A combustible material is a material that can burn (i.e., sustain a flame) in air under certain conditions.

Flammability diagrams show the control of flammability in mixtures of fuel, oxygen and an inert gas, typically nitrogen.