Also the hot smoke that is flowing out of the door is moving around the rest of the building, threatening people who are elsewhere. As the smoky gas layer in a room becomes hotter it radiates more and more heat to the rest of the furniture in the room. Finally, the whole room catches fire. This is flashover. After flashover the room continues to burn and becomes very hot.
Flames come out of windows and doors.
Fire spread to other rooms becomes likely. The fire is very difficult to control. This is extremely hot, nearly red hot. The room will carry on burning until it has consumed all the fuel, or until firefighters put it out. Since fires grow so fast and become deadly so quickly, it is important that we detect them early.