(Appeared in print as "Go Ahead — Blow Smoke")
Roughly a quarter century ago, a Bakersfield, Calif., fire chief named Cliff Allman crossed paths with an avid balloonist and entrepreneur named Dexter Coffman. Soon after they were working to develop a tactic that utilized high-pressure portable fans to assist firefighters in ventilating structures in a coordinated manner.
Today, one would be hard-pressed to find a fire department that does not use positive-pressure ventilation in one way or another. Given the fire service’s traditionally conservative approach to change, the widespread acceptance of PPV occurred at the speed of light. Indeed, the acceptance of PPV as an effective firefighting tactic by a majority of departments, in a mere 25years, borders on a miracle.
Then and Now
In the early days of PPV, building construction tactics and fire loads, as well as the fans used to “pressurize” structures back then, were much different than they are today. Back then, according to the NFPA, fires took 14 minutes from the time a smoke detector would activate until that room would flash and the blaze would begin to aggressively move through the structure. In contrast, today’s structure fires flashover in less than four minutes. This is largely due to the fact that fire loads inside modern structures primarily are made of synthetics. Fire loads in today’s structures can generate more than 15,000 BTUs of energy per pound. In contrast, fires of yesterday — when items inside our structures were made of ordinary combustibles — only would generate 5,000 BTUs per pound. Contributing to the problem is also the fact that the products of combustion that crews faced 25 years ago were much less lethal than they are today. The Centers of Disease Control and Prevention said the interior environments that today’s firefighters face are a “soup of carcinogens.”
On the other hand, while the best fans used by most fire departments 25 years ago were rated at 5,000 cfm, many of today’s fans operate at 20,000 to 30,000 cfm. When matched with ample exhaust, coordinated pressurized attacks give today’s firefighters an option for removing massive energy and lethal environment; at the same time, victims that may be in the structures may have a better chance of survival than without a coordinated attack. Without adequate exhausts, crews effectively will create convection ovens inside the buildings, which may compromise what may have been survivable spaces for victims and firefighters.
A complicating factor is that today’s structures are built with much less mass and with more engineered and adhesive-laden materials than ever before. For these reasons and more, today’s roofs are no place for crews to operate when fires are burning below them if positive-pressure attack — the use of fans to remove the products of combustion during a coordinated attack to assist crews in fire knockdown — is a viable option. It is interesting that roofs meet most of the OSHA-established criteria for defining a confined space, yet firefighters often are on them after the fire has compromised the system, after the fire has been extinguished, or when conditions are so aggressive that a large-enough hole cannot be opened to provide adequate relief for interior crews.