The most commonly cited disadvantage of positive pressure fans is that they push additional oxygen into a burning structure, which can intensify the fire and increase heat release rates. But that’s only one of several significant drawbacks. Positive pressure ventilation (PPV) fans, widely used in firefighting to clear smoke from buildings, also introduce carbon monoxide when gas-powered, create dangerous noise levels, and can push fire and toxic gases into unaffected areas of a structure.
Fire Growth and Increased Heat
Positive pressure fans work by forcing a high-volume stream of air into a structure through one opening while smoke and heat exit through another. The problem is that this incoming air is rich in oxygen, which is exactly what a fire needs to grow. Research from the National Institute of Standards and Technology found that PPV caused an increase in heat release rate for roughly 200 seconds after ventilation began. While the fire eventually declined faster than it would have with natural ventilation alone, that initial surge can be extremely dangerous for firefighters still inside the building.
In some test scenarios, positive pressure ventilation created higher temperatures at lower elevations within the structure. That’s particularly concerning because lower elevations are where firefighters crawl to stay beneath the heat layer. If PPV pushes superheated gases downward or drives fire into new compartments, it can trap crews or cause rapid flashover conditions in areas that were previously tenable. NIST has noted that PPV “has not been characterized carefully enough to establish specific guidelines for optimum use,” meaning crews are often relying on experience and judgment rather than precise protocols.
Carbon Monoxide From Gas-Powered Fans
Most PPV fans used in the fire service run on gasoline engines, and this creates a problem that’s easy to overlook: the fan’s own exhaust gets pulled into the airstream it’s pushing into the building. Testing conducted for the U.S. Fire Administration measured what happens when a gas-powered PPV fan operates in positive pressure mode on a structure that starts with zero carbon monoxide. Within two minutes, CO levels inside the building reached 58 ppm. After 12 minutes, the concentration held steady around 57 ppm.
Across 13 ventilation tests, the average CO concentration introduced and maintained inside the structure was 62 ppm. Some tests recorded levels as high as 73 ppm before leveling off around 70 ppm. For context, the threshold considered significant in these tests was 35 ppm. Every single positive pressure test exceeded that level, often by double. This means that while the fan is clearing visible smoke, it may simultaneously be filling the structure with an invisible, odorless toxic gas. Occupants who survived the fire or firefighters working without respiratory protection could be exposed to harmful CO concentrations that the PPV fan itself created.
Interestingly, the same research found that when gas-powered fans were used in negative pressure mode (pulling air out of the structure rather than pushing it in), CO levels dropped to acceptable ranges. The issue is specific to positive pressure operation, where exhaust gases get entrained in the air cone directed into the building.
Noise Levels That Impair Safety
PPV fans are loud. Decibel testing by a fire department documented for the U.S. Fire Administration recorded PPV fan checks peaking at 100 dB, comparable to standing next to a running chainsaw. Even shorter tests registered averages of 87 dB with peaks at 90 dB. These levels matter for two reasons beyond simple hearing protection.
First, high noise interferes with verbal communication. On a fireground, crews need to hear radio transmissions, shout commands, and coordinate movements through a structure. A PPV fan running at the front door creates a wall of noise that can mask critical information. Second, and more urgently, noise can prevent firefighters from hearing a victim calling for help, detecting the alarm of a downed firefighter’s emergency locator device, or noticing their own low-air warning on their breathing apparatus. Any of these missed signals can be fatal.
Uncontrolled Spread of Smoke and Fire
Positive pressure ventilation only works as intended when the air has a clear path: in through one opening, out through another. If the exhaust opening is too small, hasn’t been created yet, or is in the wrong location, the pressurized air has nowhere to go. Instead of clearing smoke, it can push fire and toxic gases into uninvolved parts of the building, including rooms where occupants may be sheltering or areas where search teams are operating.
This is one of the most tactically dangerous disadvantages. A fan started too early, before ventilation openings are coordinated, can drive fire through hallways, up stairwells, or into attic spaces. The pressure differential can also force smoke into wall cavities and ceiling voids where hidden fire spread becomes difficult to detect. Timing and coordination with interior crews are essential, but on a chaotic fireground, communication breakdowns happen frequently.
Turbulence and Air Quality Disruption
In non-emergency settings like industrial facilities or cleanrooms, positive pressure systems carry a different set of disadvantages. When pressurized air enters a space, it doesn’t always flow smoothly. The airstream leaving a supply inlet spreads outward at increasing angles, and when it encounters walls, furniture, or equipment, it generates backflow and vortex patterns. These vortices mix air layers together, stirring up settled particles and redistributing contaminants throughout the room rather than pushing them out.
In environments where particle control matters, such as pharmaceutical manufacturing, electronics assembly, or hospital operating rooms, this turbulent mixing can defeat the purpose of the ventilation system entirely. The difference between laminar (smooth, layered) airflow and turbulent airflow determines whether contaminants are swept away or simply redistributed. Positive pressure systems that aren’t carefully designed with uniform flow velocities tend to create the turbulent conditions that spread particles rather than remove them.