What indicates a structure fire in an emergency call?

Heavy black smoke coming from a window is a critical indicator of a structure fire during an emergency call. This visual cue often signifies that materials within the building are burning, which can produce thick, dark smoke due to the combustion of organic materials and other substances. The presence of this smoke can help emergency telecommunicators assess the severity of the situation and relay important information to responding units about the potential dangers involved, such as the presence of flames and the likelihood of rapid fire spread.

While other options may suggest a problem or irregularity within the building, they do not have the same level of specificity and immediacy in relation to fire emergencies. Unusual odors might point to issues like gas leaks or electrical problems but are not definitive indicators of fire. Sounds of breaking glass could indicate a fire or other forms of disturbance but lack the direct correlation to fire activity. Flashing lights could suggest emergency vehicles or other alerts but do not provide information about conditions inside the structure. Hence, heavy black smoke is the most clear, urgent indication of a fire situation that requires immediate response.