Gas Explosions


Reference: Adapted from CFBT–US (Ed Hartin)

Many older texts dealing with basic fire behavior or ventilation used the terms smoke explosion and backdraft interchangeably. However, smoke explosion or fire gas explosion and backdraft are quite different phenomena.

In the case of both backdraft and smoke explosion, smoke is the fuel. However, the other sides of the fire triangle are quite different. A backdraft requires:

  • A high concentration of fuel gas / vapour,
  • Low concentration of air, and
  • Temperature above the ignition temperature of flammable products of combustion and pyrolysis produces.

On the other hand, a smoke explosion requires:

  • A mixture of fuel (smoke) and air within the flammable range but will be below the ignition temperature of flammable products of combustion and pyrolysis products (see diagram below).
  • If the fuel / air mixture had reached its ignition temperature, it would already have ignited. In many respects, a smoke explosion is similar to ignition of propane or natural gas inside a structure.


If a source of ignition is present, the fuel/air mixture will ignite explosively. Factors that influence the violence of a smoke explosion include the extent to which the structure confines the fuel/air mixture and how close the concentration of fuel and air is to a stoichiometric mixture (ideal for complete combustion). The more confined and closer the concentration is to stoichiometric, the greater the violence of the explosion.

Smoke from an underventilated fire can flow through leakage in a structure to collect in concealed spaces or other compartments within the building. Remember, smoke is fuel! If smoke is present, even if cool and well away from involved compartments there is potential for a smoke explosion.