Three elements are required:

1. oxygen in the air;
2. an inflammable substance, mixed with air. This substance can be:
   • gas (methane, acetylene, etc.);
   • liquid (petrol, solvent, etc.);
   • solid (sulphur, wood dust, etc.);
3. an inflammable source:
   • an electrical arc or a spark with sufficient energy
   • and/or a rise in temperature.

These three elements together are sufficient to trigger an explosion which may be transferred to the environment.