Smoke, fire spread and route decisions

Most people who die in building fires are killed by smoke rather than by flames. Smoke spreads faster and further than fire, carries toxic gases, and cuts visibility so badly that people lose their way in corridors they walk every day. HSE-105 explained the fire triangle and the classes of fire. This lesson looks at fire and smoke from the point of view of a warden who must decide, in seconds, whether a route is safe, whether to redirect people, and when to stop and protect people in place. Wardens do not need to be fire scientists, but they do need a working picture of how quickly conditions change.

How a room fire develops

A fire in a room usually passes through recognizable stages. In the growth stage, a small fire heats the fuel nearby and spreads. Hot smoke rises and forms a layer under the ceiling, which grows deeper and hotter. If enough fuel and air are available, the hot layer radiates heat downward until all the exposed fuel in the room ignites almost at once. This is flashover, and after it nobody in the room can survive. In ordinary furnished rooms with modern plastics and foams, flashover can happen within a few minutes of ignition, much faster than in rooms furnished decades ago.

After flashover, the fire is fully developed and burns at the rate allowed by the air coming in through doors and windows. Flames and smoke push out of the room into the corridor. If doors are closed, fire separations hold the fire back for a period, and if sprinklers operate early, the fire is usually controlled long before flashover. That is why closing doors and sprinkler protection are the two most important features for a warden to understand.

How smoke moves

Hot smoke is buoyant. It rises to the ceiling, spreads along it, and moves into corridors, stairs, shafts and any opening it finds. In tall buildings, the stack effect, caused by the difference between indoor and outdoor temperatures, can drive smoke up shafts and stairs in cold weather and, more weakly, down in hot weather. Wind, ventilation systems and open doors also move smoke. A single door left wedged open can let smoke fill a stair that would otherwise protect hundreds of people.

As smoke cools, it loses buoyancy and drops lower. A corridor that had a clear layer of air below the smoke a minute ago can fill to the floor. Visibility is the first thing to go: when people can no longer see an exit sign 10 m away, they slow down, hesitate, and turn back. The gases then begin to affect them. Carbon monoxide reduces the blood's ability to carry oxygen, and hydrogen cyanide, from burning plastics, foams and wool, acts even faster. Both cause confusion and collapse before the person realizes they are in danger.

Fire separations and protected routes

Buildings are divided by fire separations, walls and floors built to resist fire for a set period, with fire doors that close and latch. Exit stairs are enclosed by fire separations so that once people are inside, they are protected from fire on the floors. Corridors in many buildings are also separated. These protections work only if doors are closed, latched and undamaged, and if holes made for cables and pipes have been properly sealed. A fire door wedged open, or a ceiling tile removed above a fire wall, can defeat the whole design.

In the US, fire barriers, smoke barriers, fire doors and exits are set by the International Building Code, NFPA 101 and local codes, and the fire code requires them to be maintained, with fire doors inspected under NFPA 80. Wardens should report damaged doors, missing closers and open penetrations during their inspections, as described in lesson 4.

Deciding whether a route is safe

The warden's decision is usually simple: go away from the fire and smoke, by the nearest protected route. The plan should give primary and alternative routes for every area. When the primary route is affected, the warden switches to the alternative and makes sure others do too. A warden should never send people through smoke when another route exists, and should never send people toward a fire to reach a familiar exit.

Where no route is safe, such as when both stairs are affected or when people on an upper floor are cut off, the right action is to protect people in place until rescued. Wardens should move people into a room as far from the fire as possible, close the door, seal gaps with cloth or tape if available, call the emergency number to give the exact location, and signal from a window. Breaking windows is usually a mistake because it can draw smoke in and cannot be undone. People should stay low if smoke enters, where the air is cooler and cleaner.

Protect in place, defend in place and shelter in place

The phrase "protect in place" can mean different things. In this lesson it means moving people to a room away from the fire and protecting them until rescue, because they cannot leave safely. In health care and care homes, "defend in place" means keeping patients or residents in their rooms or moving them horizontally to a safe compartment behind a fire separation on the same floor, because moving them down stairs could harm them. In high-rise residential buildings, occupants are often told to stay in their units unless the fire is in their unit or threatens it. "Shelter in place" is also used for outdoor hazards such as chemical releases, where people stay inside with doors and windows closed and ventilation off. Wardens must know which strategy their building's plan uses for which situation.

Horizontal evacuation

Many hospitals, care homes and large buildings are divided into compartments separated by fire and smoke separations. Moving people horizontally through the separation into the next compartment protects them quickly without using stairs. A horizontal exit, a door through a fire wall to an area of equal safety, can count as an exit in building codes. Wardens in such buildings must know where the compartment boundaries are, how many people the next compartment can hold, and when to move further.

Using extinguishers as a warden

HSE-105 covered extinguisher selection and the PASS technique. For wardens, the important decision is whether to try at all. A warden should consider using an extinguisher only if the alarm has been raised, the fire is small and not spreading, the warden is trained, the extinguisher is the right type, the warden has a clear exit behind them, and the room is not filling with smoke. If any of these is not true, the warden closes the door and leaves. A warden whose job is to lead an evacuation should usually leave firefighting to others, because the time spent fighting the fire is time not spent getting people out.

Special hazards

Some fires behave differently. Lithium-ion battery fires, from e-bikes, scooters, laptops and power tools, can flare violently, give off toxic gas and reignite after they seem to be out. Fires involving gas cylinders, flammable liquids or chemicals may spread very quickly or explode. Electrical room fires and kitchen fires with cooking oil need special extinguishers. In all these cases, wardens should clear the area, close doors and tell the fire service exactly what is involved. Lesson 4 described how wardens can reduce these risks through housekeeping and reporting.