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One of the most dangerous situations firefighters can face is a rare but powerful effect called a backdraft. Although definitions of a backdraft vary somewhat, in general it is a sudden explosion of hot gases as fresh oxygen enters a smoldering airtight room. Professional firefighters are trained to recognize signs of a potential backdraft, but even the most experienced firefighter can be caught by surprise.
Fire requires both fuel and oxygen to continue burning, so if a fire breaks out in a closed area, it may use up the entire supply of oxygen before running out of flammable objects as fuel. The room would still be extremely hot, but there would be very few visible flames. Instead, a thick cloud of smoke, often yellow or brown in color, would fill the room. Firefighters recognize the presence of yellow or brown-colored smoke as a sign of incomplete burning. This is a classic warning sign of a potential backdraft.
Since the fire is starving for oxygen to continue burning, the air pressure inside the room can be noticeably different from the air pressure outside the room. The fire wants to draw in more oxygen through any available opening, which often results in puffs of smoke suddenly appearing and disappearing around the door or window frames. The difference in air pressure may also cause windows outside the room to rattle or vibrate. These are other signs of a potential backdraft firefighters are trained to observe.
If the door to a burning airtight room is opened suddenly, a fresh supply of oxygen is delivered straight to the smoldering fire. The heat of the room also heats up volatile gases, and the result is often a powerful explosion and instant flames. This is the dreaded backdraft, and it can injure or kill anyone entering the room. Thankfully, backdrafts are rare events, but it only takes one to restart a fire or cause more structural damage.
If a potential backdraft is suspected, firefighters can use tools to ventilate the room from the highest point possible, such as the top of a door. This allows the heated gases to escape without drawing in enough fresh oxygen to create combustion. Once the hot gases have been ventilated, the chances of a backdraft forming are reduced significantly. The fire may still reignite with additional oxygen, but a catastrophic explosion of gases is much less likely.