Why are combustible and flammable gases and vapors dangerous?

Atmospheres containing combustible or flammable gases or vapors can be dangerous because of the threat of fire and explosion. Three ingredients are necessary for an atmosphere to become flammable or explosive: an ignition source (heat or flame), fuel (combustible gas or vapor), and oxygen. However, the proportions of fuel and oxygen in a mixture must be within the flammable range for this mixture to be readily ignitable.

Remember, the atmosphere inside a confined space can change rapidly and unexpectedly. Also, any ignition source (such as sparks from grinding or welding equipment, static electricity, or unapproved electrical equipment that is not non-sparking or even smoking) can initiate an explosion.