Smoke is a mixture of airborne solids (soot, fibers, dust), liquid particles (hydrocarbons, water), and gases (like CO, HCN, and other toxic gases), often produced by pyrolysis or combustion reactions.

Firefighters must understand the properties of smoke to assess fire situations effectively.

Buoyancy

Buoyancy is the upward force applied to a mass that has a lower density than its surroundings. Thermal buoyancy occurs when air and smoke become less dense as their temperature rises, causing the warm air or smoke to rise when in contact with cooler surrounding air.

Due to thermal buoyancy, smoke moves through openings in walls or ceilings above the fire. Though buoyancy itself is relatively small, it drives the movement of smoke. In tall buildings, buoyancy’s effects are more pronounced, and mechanical smoke control systems may be required. However, as smoke moves away from the fire, its temperature drops due to heat transfer and dilution, reducing buoyancy’s impact.

When smoke rapidly exits a fire area, it indicates the gases are extremely hot. Even dense substances become lighter than air when sufficiently heated. On the other hand, cooler smoke tends to rise slowly and may even descend as it cools. Low buoyancy suggests that the temperature inside the room of origin is relatively low.

Expansion

In addition to buoyancy, heat from a fire can cause smoke to move through expansion. As shown below, the energy released by the fire heats the smoke, lowering its density and increasing its volume. If the space is confined, pressure rises.

In typical fires, the air volume can increase more than threefold due to heating. Since air is composed of about 78% inert nitrogen, it is nitrogen expansion that causes this significant volume increase. Even a slight pressure rise inside a fire room can create a pressure difference with sealed areas, forcing large amounts of smoke to leak out.

Volume

When assessing a fire, smoke is often the only visible source of information. The amount of smoke provides clues about the size, location, and stage of the fire inside a building. However, the volume of smoke isn’t always reliable. As shown in the figure, heated smoke rises vertically, and if vertical movement is restricted, it spreads horizontally. When smoke encounters horizontal obstacles, it deepens and may flow into other vertical paths, such as gaps or attics, appearing far from the fire’s origin. According to recent studies from organizations like the U.S. National Institute of Standards and Technology (NIST), most residential fires occur in ventilation-limited conditions, meaning there may be little visible flame from the outside due to lower temperatures and smoke production. Firefighters often find that after seeing a building emit a lot of smoke, the actual fire source is smaller or in an unexpected location. As with all fire information, it’s crucial to consider the potential for fire spread inside the building and not rely solely on one piece of information.

Speed

The speed of smoke reveals information about the pressure building up inside a structure. Turbulent or fast-moving smoke indicates high-temperature gases.
The speed of smoke during a fire typically ranges from 0.3 to 1.0 m/s horizontally and 3.0 to 5.0 m/s vertically, but these rates can vary greatly depending on changes in volume inside the building.

Neutral Plane

In a burning building, a boundary forms where the hot rising smoke meets the incoming fresh air from outside, called the “neutral plane.”

Raising the neutral plane can be achieved by ventilating the building. A gradual lowering of the neutral plane indicates the accumulation of hot, flammable gases, which, if not managed, can lead to a flashover. A sudden drop in the neutral plane suggests rapid fire progression, signaling the need for emergency evacuation from the hot zone.

As a fire progresses toward a flashover, the neutral plane (smoke layer) lowers.

Color

Smoke color varies depending on the material, type, and conditions of combustion, with some general principles.

• Black smoke: Indicates a rich supply of fuel due to limited air supply. Solid particles are mixed into the smoke, causing its black color.
• Brown smoke: Often seen in the early stages of pyrolysis in wood products, due to the release of tar. Brown smoke can also indicate the presence of fire retardants.
• White smoke: Lacks the solid particles found in black smoke and often resembles steam, as it’s typically caused by water evaporation. Though it may seem less harmful, white smoke can still contain dangerous levels of fuel vapor, so it should never be dismissed as safe.

This version simplifies the technical language to make the key concepts clearer while maintaining accuracy.

Reference Video

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