What is a likely indicator of significant fire danger in a structure?

Black smoke is a significant indicator of fire danger in a structure because it usually indicates that the fire is burning a high-temperature, high-intensity fuel, such as plastics, rubber, or other synthetic materials. This type of combustion typically releases a large amount of unburned carbon particles, which contribute to the black coloration of the smoke. The presence of black smoke often signals that the fire is producing toxic by-products that can pose a serious risk to both firefighters and occupants. Additionally, heavy black smoke is commonly associated with deep-seated or rapidly spreading fires, indicating that immediate and aggressive firefighting responses may be necessary to control the situation and prevent further danger.