When passengers see a white haze drifting from the air-conditioning vents, they often assume the cabin is filling with smoke. The white smoke-like thing is harmless water vapor generated by the aircraft's air-conditioning system. The mist appears briefly while the packs cool humid cabin air, then disappears as the airplane climbs and the system steadily removes water vapor and liquid. Federal regulation requires manufacturers to prove that under normal operations the interior remains free of harmful concentrations of smoke, vapor, or toxic fumes, yet no filtration is applied to the bleed air that is drawn directly from the engine compressors. Because of that design, oil fumes from a hot section of the engine or APU can sometimes travel into the cabin. While HEPA filters downstream capture 99.97% of particulate matter, they do not remove gases and vapors, so any oil mist that reaches the air supply will not be trapped.
What is the smoke in an airplane cabin?
The thick white smoke-like thing in the cabin is mist or vapor that fills the aircraft when taking off from humid places. This occurs due to the difference in temperature between air inside the aircraft and outside air at the humid airport, causing invisible water vapor to condense.
The white smoke-like substance is harmless water vapor from the aircraft's air conditioning system. Most planes are equipped with a water separator to prevent this from happening often, yet the event is still very common. The mist forms because air from the pack is cold: cold air plus humid air equals mist. This mini-cloud disappears quickly as the aircraft climbs and the cabin humidity equalizes. HEPA filters remove 99.97 percent of particulate material from recirculated airflow, but they do not affect water vapor. Water vapor is not smoke and contains no solid particles, so it passes through unchanged and remains harmless.
What causes vapor in an airplane cabin?
Water vapor inside an airplane cabin is caused due to condensation of humid air. Water vapor is a natural part of air, and inside an aircraft its state is controlled by an air-cycle refrigeration system that feeds conditioned air at roughly 40°F (4.4°C) and very low humidity. The cabin humidity is low - about 20 % relative humidity on most jets, and as little as 2-3% in the cockpit - because bleed air pulled from the engines is dry. This low level is intentional: low humidity dehydrates passengers and crew but is suitable for the metal aircraft are built from and prevents condensation.
Fog that occasionally appears in airplane cabins is a result of condensation. When the airplane is grounded at a humid location the air outside holds a lot of moisture as vapor, and while the cabin door is open warm moist air from the outside fills the cabin. The instant the packs start, cold dry supply air mixes with that warm humid cabin air, lowering the mixture to its dew point. At any surface or vent where the local temperature falls below the dew point, water vapor condenses into visible water droplets, so condensation appears near the AC inlets and mist curls through the cabin. Once the doors are closed and the AC system removes moisture from the cabin air using water separators. At that stabilized level, condensation does not occur, so the fog you see before takeoff is that last step of the process and vanishes within minutes.
Is smoke in an airplane cabin dangerous?
Yes, smoke in an airplane cabin is dangerous. Irritant smoke gases induce tears, pain, and disorientation. Inhalation of toxic gases in smoke is the primary cause of fatalities in most fires. Smoke gases do not need to reach lethal levels to impair pilot performance, and sublethal exposures cause experienced pilots to make potentially fatal mistakes.
The committee strongly recommends cabin-fire instructional material emphasize the need to turn on all available air packs to full volume if a cabin fire or smoke is present, while industry practice specifies turning off recirculation systems if the equipment includes this option.
Are airplane cabin fumes harmful?
Yes, cabin fumes are potentially harmful. Toxic fumes have been reported inside some airplane cabins when bleed air is contaminated by engine oil, hydraulic fluid, anti-icing fluid, or other potentially hazardous chemicals. These fumes contain volatile organic compounds like toluene and xylene that irritate the eyes, throat, and lungs, carbon monoxide that deprives the brain and organs of oxygen, and organophosphates like tricresyl phosphate which are potent neurotoxins.
Passengers and crew will experience immediate symptoms: eye, nose or throat irritation, confusion, dizziness, headache, nausea, shortness of breath or coughing, tremors or loss of coordination, fatigue and muscle weakness. Long-term exposure leads to aerotoxic syndrome: memory loss, brain fog, difficulty concentrating, chronic weakness, pain, asthma, chronic bronchitis, depression, irritability and sleep disturbances, neurological issues like tremors, tingling, balance problems and neuropathy, and mood and behavior changes.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
