Fire Phases & Interior Hazards

FIRE PHASES

As a fire progresses, different phases can be observed if it's left to burn freely. Having a strong knowledge of these phases will save your life if you recognize flashover conditions before they occur. If you have read this far into my article, make sure you take the time to learn these.

Ignition Phase- The phase of fire development where the fire is limited to the point of origin

Growth Phase- The phase of fire development where the fire has moved from the point of origin and has begun to involve other fuels. Thermal layering takes place (cooler air is pushed downward, hotter air rises).

Fully Developed Phase- The phase of fire development where the fire is free-burning and consuming large amounts of fuel

Decay Phase- The phase of fire development where the fire has consumed the available fuel or oxygen, and is shrinking in size

Other than their simple definitions, there are a few things to know. Flashover takes place between the Growth and Fully Developed phases. This is the point where every combustible material in the vicinity will spontaneously ignite.

Fires start small and grow outward, this much I am sure of. Spurred by its self sustaining chemical reaction, fire will always try to grow. If that fire is in a room, it might not just grow outward gradually. At some point, it will generate enough heat in a room to bring the entire room to a temperature where the fire itself doesn't need to directly contact fuel to ignite it anymore.

This is called flashover, and it's another situation you're better off seeing in movies than through your SCBA mask. Every smoke particle in the air will burn, the walls, furniture, everything all at once. Temperatures in excess of 1,000ºF are common. Visit the Fire Suppression page for information on preventing and surviving a flashover.

STRUCTURE FIRES

Structures trap heat and smoke. It's also much harder for a firefighter to get into a 3rd story apartment and put out a fire than a trash can fire out on the street. These two facts alone make interior structure fires incredibly dangerous. Fire also has a tendency to hide within the walls and continue to spread. People don't like it when the Fire Department puts out a fire in their house, only to have it rekindle a few hours later.

Lucky for us, most fires in buildings will only be contents (meaning the structure itself is not on fire, merely the materials within the structure like a couch or Thanksgiving Dinner). As our society continues to create more and more household products and furniture with new plastics, polyvinyl materials, latex, acrylics (and Lord knows what else), the byproducts from a contents fire will continue to become even more toxic.

The fuel load refers to the total amount of combustible material within a certain area. This will (obviously) determine almost everything about the fire and its ability to spread.

Within an interior structure, fire will eventually rollover (also called flameover). It's a condition where products of combustion (smoke, heat) accumulate on a ceiling to a concentration where they ignite momentarily. When a ceiling becomes hot enough to ignite the smoke like this, you're being given a strong indication that flashover is imminent. If you come across this situation make sure to change the environment very, very soon.

Another potential situation is known as backdraft (also called smoke explosion). In this situation, fire won't necessarily be visible. Watch for thick carbon deposits on windows and puffing smoke through door cracks and eaves (the building may appear to be "breathing"). These are sure signs that you're looking at backdraft potential. Do not use horizontal ventilation in this situation unless absolutely necessary.

The only thing lacking from the Fire Tetrahedron in a backdraft situation is oxygen, which is exactly what you will provide if you break open a window or door. Before you do anything, look for the most common symptoms of backdraft; smoke puffing from the eaves, not flowing ("the building is breathing"), smoke stained windows, etc. Whenever possible in this situation, use vertical ventilation to give the heat a place to escape. You will have to introduce oxygen to the environment eventually, at least you will be removing heat at the same time by doing this. You'll also help prevent the fire from immediately spreading to other parts of the structure if ventilated correctly.

Are you still here? Step away from the computer and go have some lunch. Call the family, pay the bills, enjoy the weather a little bit, then come back here for more information on firefighting.