The fire alarm is given in the building without fire smoke exhaust windows
◎ 火警发生时开始产生烟雾。 Smoke is generated by fire.
◎ 在火焰的周围产生低压区， 浓烟从天花板向低压区蔓延。
Low pressure areas occur around the flames and the heavy smoke spreads from the ceilin to the low pressure areas
◎ 此时浓烟弥漫在室内， 给逃生者带来了危险， 使逃生者无法逃生或被营救。
The rooms are full of heavy smoke，which will bring danger fo the people so that they can't escape or be rescued.
The difference between indoor pressure and out-door pressure caused by high temperature will make the building collapse.