Four basic ingredients must be present for an explosion to occur in air: a flammable substance, oxygen (air), an ignition source, and dispersion. A fifth, containment (e.g., room, mine shaft, silo) magnifies and directs damage.

Flammable substances and oxygen must be sufficiently concentrated to form the explosive atmosphere, and the flammable substance must be dispersed or mixed well enough in that atmosphere.

Ignition sources vary and may include hot surfaces from various sources (e.g., coils, resistors, lamps, brakes, hot bearings, and welding or cutting torches). Electrical sparks are another source. Friction or impact sparks may be other sources, as are open flames and heated gases. Spontaneous chemical or biological processes, lightning, intense electromagnetic radiation, ionizing radiation, adiabatic (no heat loss) compression, and shock waves are other potential sources.

Flammable Substances Vary

Gases, liquids, or solids may be flammable substances. A flammable gas may be a hydrocarbon, such as methane (CH₄) or propane, that requires little energy to react with oxygen in air.

Flammable liquids also may be hydrocarbon compounds (e.g., solvents, gasoline) that evaporate at room temperature to form potentially explosive conditions. The flash point-the lowest temperature at which sufficient vapor volume will be created to permit ignition under specific conditions- is very important: high-flash-point liquids are less hazardous than low-flash-point liquids. Mists of flammable liquids also can create explosion hazards.

Dusts and fibers (including fluff materials) may be flammable solids. Particle size and surface characteristics are important to explosion dynamics: finely divided and dispersed solids may present higher explosion hazard.

Coal dust is one example of those dynamics. For example, the explosion of coal-mine methane could trigger a coal-duct explosion from the dust already in the air or that is settled but reentrained in the air by the methane explosion's shock wave. These dust explosions often have more severe consequences.

Flammable materials have lower explosion limits (LELs) and upper explosion limits (UELs). The explosive range lies between the LEL and UEL. Diluting flammable materials concentrations and oxygen starvation are two ways of extinguishing fires. While uncommon in manufacturing, oxygen deprivation through the sealing of mine portals and ventilation shafts has been used frequently to extinguish coal-mine fires.