One of the most hazardous conditions a fire- fighter will ever encounter is a backdraft (also known as a smoke explosion). A backdraft can occur in the hot-smoldering phase of a fire when burning is incomplete and there is not enough oxygen to sustain the fire. Unburned carbon par- ticles and other flammable products, combined with the intense heat, may cause instantaneous combustion if more oxygen reaches the fire. Firefighters should be aware of the condi- tions that indicate the possibility for a backdraft to occur. When there is a lack of oxygen during a fire, the smoke becomes filled with carbon diox- ide or carbon monoxide and turns dense gray or black. Other warning signs of a potential back- draft are little or no visible flame, excessive heat, smoke leaving the building in puffs, muffled sounds, and smoke-stained windows. Proper ventilation will make a backdraft less likely. Opening a room or building at the highest point allows heated gases and smoke to be released gradually. However, suddenly breaking a window or opening a door is a mistake, because it allows oxygen to rush in, causing an explosion. Which of the following is NOT mentioned as a potential backdraft warning sign?