Which type of smoke suggests that the contents of a fire are transitioning to a structure fire?

Brown smoke indicates that the contents of a fire are transitioning to a structure fire. This type of smoke is often produced when synthetic materials, such as plastics and treated wood, are burning. When these materials ignite, the combustion produces a dense, brown-colored smoke that suggests the fire is reaching a more complex phase, potentially entering a structure or involving a wider array of materials.

The presence of brown smoke often signals a significant change in the fire dynamics and can be a warning for firefighters to prepare for an escalation. It can also indicate poor combustion due to a lack of oxygen, which is common during the initial phases of fire spread in enclosed environments.

In contrast, black smoke is typically associated with the burning of hydrocarbons and indicates a fully developed fire with intense heat and energy release. Yellowish-gray smoke may suggest a smoldering fire or a mixed combustion of materials. Clear smoke typically indicates that the fuel is burning relatively cleanly, often coming from non-combustible sources, and does not suggest an impending structural fire.