THE BACKDRAFT A New Look at an Old Danger
FIRE REPORTS
Photo by John E. Bowen
Heavy smoke was pushing from around first-floor windows and doors of the two-story brick rowhouse as Baltimore City, MD, firefighters arrived at the incident scene. Despite the heavy smoke, no fire was visible from outside.
Lieutenant Nelson Taylor of Engine Company 8 and two firefighters with a pre-connected handline crawled into the kitchen through a rear door in an effort to locate the fire. They had made their way approximately five feet into the smoke-filled room and were nearing an interior stairway leading to the basement when an “explosion” occurred, causing the basement to erupt into flames.
A typical backdraft situation you’re thinking? Oh, it was most certainly a backdraft, but, no, it was definitely not the typical backdraft. You see, Lt. Taylor was pulled or “sucked” into the flaming basement by the sudden events, not blown outward! He was knocked off balance and was literally carried down the stairs and into the developing backdraft by the inrushing air. The firefighter following behind the lieutenant was blown backward into the kitchen in what we might describe as the “conventional” backdraft.
Lt. Taylor was severely burned over 65% of his body and suffered a head injury when he fell down the stairs. He was transported by ambulance to the shock-trauma facility at University Hospital, and later transferred to the Francis Scott Key Medical Center burn unit where he succumbed to his injuries about 12 hours later.
Firefighters Morris Hedges and Richard Watson were also burned about the face and hands. Both were treated and released at the fire department’s infirmary. A resident of the fire building also received firstand second-degree burns to his hands while trying to extinguish the fire before firefighters arrived.
The backdraft has always been a threat to firefighters working at structural fires, of course, especially in the winter months when buildings are apt to be tightly sealed against the cold weather. So, let’s take a look first at what have become known in the fire service as the “typical” conditions that lead to a backdraft. Then we will come back to the Baltimore incident.
TYPICAL BACKDRAFT CONDITIONS
The normal atmospheric oxygen concentration is just under 21% at sea level and it decreases slightly with increasing elevation. You know, of course, that oxygen (O2) is necessary for and is consumed during combustion. The O2 reacts chemically to form carbon dioxide (CO2) if abundant O2 is available, and carbon monoxide (CO) if O2 availability is significantly below the usual 21%. A free-burning fire in a tightly closed area (a room or a basement, for example) soon lowers the ambient oxygen concentration to a level considerably less than 21%. When the oxygen level drops to about 15%, flaming combustion ceases, the fire begins to smolder, and large quantities of CO are produced.
Carbon monoxide, you will recall, is both markedly toxic and highly flammable. For example, a concentration of 0.64% (6,400 ppm) of CO in the air produces a blood saturation of 80% in most people within a minute or two. If the CO content of the air doubles to 1.28%, two or three breaths will likely render a person unconscious. Death usually follows within one to three minutes under the latter conditions.
The ignition temperature of carbon dioxide is 1,204°F (651 °C); its lower and upper flammable limits are 12.5% and 74%, respectively.
Three factors must come together simultaneously for a backdraft to occur:
- A large amount of fuel that has already been heated to its ignition temperature (the CO that has
- been produced by the smoldering fire);
- A source of ignition (the smoldering fire itself);
- A suddenly increased availability of O2.
Photo by John E. Bowen
When these three components of a backdraft are brought together in an enclosed area, ignition of the fuel, the CO, occurs virtually instantaneously and with explosive rapidity.
We have been taught for decades that the proper method to minimize the potential for a backdraft is to ventilate the enclosed area, be it a basement or cellar, a room or an entire building, at the highest point. This permits the upward and outward flow of smoke, CO, and the other heated combustion products, without allowing an influx of cooler air and, specifically, O2. You must be certain that these combustion products are venting to the outside, that the outward flow is not being blocked, perhaps by a suspended ceiling or interior partition, for example.
There is no doubt that these superheated, explosive gases will ignite when finding their needed air supply. Fire strategy demands that we choose where the fire triangle will be completed. If the gases are channeled vertically, the O2 will be found in free space and the energy of combustion will be unleashed rather harmlessly in air. However, it is when the necessary O2 is “sucked” into an enclosed space that the same energy, now confined by the structure, causes the damaging backdraft phenomenon.
Photo by John E. Bowen
Narrow (30°) fog streams can be introduced into the fire area once effective ventilation has begun. Water application must be very carefully coordinated with ventilation though, ensuring that ventilation at the highest point has been started first. But don’t wait too long to start the water after ventilating or you will suddenly have a large free-burning fire on your hands.
At this point, let’s examine more closely the circumstances that led to the tragic event in Baltimore.
The two-story structure was a rowhouse, the end unit of a blocklong series of interconnected dwellings. It was constructed of brick but the brick had been covered with vinyl siding.
The dwelling had been built on a gently-sloping parcel of land so that the first floor was approximately five feet above grade in the front and at ground level in the rear. This placed the basement about one-half below grade in the front and almost fully below grade in the rear. There were very small basement windows in the west wall and a larger one in the front; the rear and east basement walls were without any windows.
Basements in dwellings frequently have a heavy fuel load because they are used for storage, and this one was no exception. In fact, this fire was found subsequently to have started in a rolled up and stored mattress. The cause of ignition was probably children playing with matches or possibly careless smoking.
A fire that originates in a basement, by virtue of its partial below-grade location in a structure and the storage function that the area often serves, can be expected to generate great quantities of smoke and carbon monoxide. After all, the below-ground and usually tightly-sealed basement is poorly ventilated at best, so O2 levels in the air are going to drop quickly once the fire starts, a condition highly conducive to CO accumulation and to backdraft. It is imperative that such a situation be recognized by the first firefighters to reach the scene and that the proper actions be taken to minimize the risk of a backdraft.
The proper procedure for attacking a smoldering basement fire that is generating great volumes of smoke is exactly the same as that for minimizing the backdraft potential in any other type of occupancy. First, establish vertical ventilation at the highest point and then get a hoseline in position as quickly as possible to protect vertical openings—the interior stairway between the kitchen and basement, for example—to prevent upward extension of the fire. The need for self-contained breathing apparatus (SCBA) is obvious, of course.
ATYPICAL BACKDRAFT
This is precisely what Lt. Taylor and his team were trying to do, locate the fire and get a handline into position at the interior staircase. Here, then, is where the situation at this particular fire began to deviate from the usual events leading to a backdraft.
The firefighters in the kitchen, having determined that the seat of the fire was in the basement, approached the door to the basement stairs, fully anticipating that they would be met by heavy smoke and superheated gases venting upward toward them when the door was opened. Instead, there was an immediate flow of air from the kitchen, down the stairway, to the fire in the cellar. It seems probable that the fire had vented itself, probably through one or more of the basement windows, just as the firefighters were opening that fateful door upstairs.
An outward flow of smoke and heated gases from the windows was thus established as the door was being opened. The suddenlyopened doorway in the kitchen then provided the channel for cool air to enter, and a replenishing (⅝ supply, to flow into the fire area. It was this flow of cool air that knocked the firefighters off balance, causing Lt. Taylor to fall or to be swept into the burning basement.
In effect, then, this incident was the reverse of the “typical” backdraft. The primary ventilation was probably accomplished through a basement window, the lowest point, when the fire self-ventilated. The interior stairway from the kitchen above then became the channel for air—and O2—to flow into the fire.
This incident was certainly unusual in the manner in which it developed, but it does serve to underscore a very important fact in the fire service. That is, nothing can ever be interpreted as “ordinary” or “predictable” in firefighting. There is always the omnipresent exception lurking around some corner, an exception that threatens every firefighter with bodily injury and death.
