Cabin air explained: bleed air, HEPA filters and humidity on planes

Barely have you sat down on the plane when your throat starts to scratch and your eyes dry out, and at the latest on a long-haul flight you reach for the water bottle. At the same time, the myth persists that on a plane you breathe “used” air that is just pumped around in circles. Both have to do with the way airliners produce their cabin air. This article explains where the air in the cabin comes from, what bleed air and packs have to do with it, how well HEPA filters really work, why it is so dry – and why the Boeing 787 plays a special role here. For spotters, there are also a few details you can recognize from the outside of the aircraft.

Cabin air: the short answer

The air you breathe on board is a mix of fresh outside air and filtered recirculated air. On almost all jets, the outside air is tapped from the engine compressor, hot and under pressure. Packs in the fuselage cool it down, a mix manifold combines it with recirculated air that has run through HEPA filters, and ducts above the overhead bins distribute it all through the cabin. At the same time, air flows out through a valve in the rear. This constant flow takes care of pressure, temperature and air exchange all at once.

Bleed air: air from the engine

A turbofan engine sucks in huge amounts of air and compresses part of it in several stages before kerosene is injected and burned. At certain stages of this compressor there are bleed ports where a small share of the compressed air is tapped off. This air is called bleed air.

Important to understand: the extraction point is upstream of the combustor. Bleed air is therefore completely normal outside air, just heavily compressed and hot as a result. In normal operation, it does not come into contact with exhaust gases or kerosene. Depending on the power setting, the system uses a low- or high-pressure stage: at high engine power, the pressure from an earlier stage is enough, and on descent with little thrust, it switches to a later stage.

After control valves and a precooler in the engine pylon, bleed air is typically still around 200 to 250 degrees Celsius with a gauge pressure of about 275 kilopascals. That is far too hot for the cabin. At the same time, this compressed air is an all-purpose tool on the aircraft:

• Cabin: pressurization and air conditioning – the most important task.
• De-icing: Hot bleed air keeps engine inlets and, on many types, the wing leading edges free of ice in flight. De-icing on the ground is a topic of its own, see De-icing aircraft.
• Engine start: Compressed air drives the starter, for example from the auxiliary power unit or an engine that is already running.
• Minor tasks: Among other things, pressure for hydraulic reservoirs and water tanks.

On the ground, when the engines are not running yet, the auxiliary power unit in the tail usually takes over the supply of bleed air – or an external air conditioning unit at the gate. How the APU, ground power and preconditioned air work together is explained in the article APU, ground power and PCA.

The packs: from furnace to cabin air

The conditioning is done by the packs (the term is often spelled out as “Pneumatic Air Cycle Kit”), on many types two of them in the wing-to-body fairing under the center fuselage. Some widebody jets such as the Boeing 747, Lockheed L-1011, DC-10 and MD-11 have three.

The heart of every pack is the air cycle machine. It uses the air itself as the refrigerant, with no refrigerant circuit like in a refrigerator. The principle in short:

1. A heat exchanger gives off heat to cold ram air that flows in from outside through an air inlet on the belly of the fuselage.
2. A compressor on the shared shaft raises the pressure once more, the air gets warm again and is cooled again.
3. In a turbine, the air expands, does work (it drives the compressor) and cools down sharply in the process – partly below freezing.
4. A water separator removes the moisture that condenses out along the way.

Through a temperature control valve, part of the hot air is routed around the machine and mixed back in afterward. This gives exactly the temperature the system requests.

Mix manifold and recirculated air: why half comes back

Behind the packs, the cooled fresh air ends up in the mix manifold. There it is combined with recirculated air from the cabin. On almost all modern airliners, the ratio is roughly 50 percent outside air to 50 percent recirculated air. The CDC quotes a recirculated share of 35 to 55 percent for large airliners.

Why recirculated air at all? Because every portion of bleed air costs energy: the engine has put work into compressing it, which is then no longer available for propulsion. Recirculated air, on the other hand, is already at temperature, under pressure and contains slightly more moisture than fresh outside air. That saves fuel and makes the air more pleasant.

So that nobody is shortchanged, there are clear minimum values. The US design rule for airliners (14 CFR § 25.831) requires at least 0.55 pounds of fresh air per person per minute in normal operation, that is, about 250 grams. For carbon dioxide, a limit of 0.5 percent by volume, converted to sea level, applies in passenger and crew areas.

In addition, there is trim air: a small share of hot air is diverted before the packs and mixed into the individual cabin zones separately. That is why the crew can set the temperature in the cockpit, in the forward cabin and in the rear differently.

How the air flows through the cabin

The conditioned air exits through nozzles along the overhead bins, flows down across the cabin and is sucked off again near the floor at the sidewalls. The main flow therefore runs from top to bottom and from the center to the sides rather than lengthwise through the whole fuselage. Part of the extracted air goes to the recirculation filters, and the rest leaves the aircraft through the outflow valve, usually at the rear bottom of the fuselage.

This valve is at the same time the regulator for cabin pressure: the wider it opens, the more air escapes and the lower the pressure. At cruising altitude, it is almost closed. You can find more about pressure and what it does to your ears under Cabin pressure and ear pressure.

Ozone: the invisible problem at high altitude

At cruising altitude, especially on routes at high latitudes, outside air can contain significantly more ozone than on the ground. Ozone irritates the airways and eyes. The US design rule § 25.832 therefore sets limits: above flight level 320, never more than 0.25 ppm in the cabin, and above flight level 270, no more than 0.1 ppm on average over any three-hour period. Manufacturers have to demonstrate that the aircraft meets these values – in practice, usually with catalytic ozone converters in the supply air path that convert ozone into ordinary oxygen. Why jets fly so high in the first place is explained in the article Why planes fly at around 10,000 meters.

HEPA filters: what they can do and what they cannot

Before the mix manifold, the recirculated portion passes through HEPA filters (High Efficiency Particulate Air). According to the US health agency CDC, they retain 99.97 percent of particles of 0.3 micrometers and up, including bacteria, fungi and viruses attached to droplets or particles. The size of 0.3 micrometers is no coincidence: it is considered the most difficult particle size to remove. Smaller and larger particles are retained even better.

Component	Filtered by HEPA?	What helps instead
Dust, fibers, pollen	yes, very reliably	–
Bacteria and fungal spores	yes	–
Viruses on droplets or particles	yes	distance and hygiene in the immediate surroundings
Odors (food, perfume)	no	high air exchange rate, additional odor filters on some types
Gases such as CO₂	no	fresh air supply, limit of 0.5 vol. %
Ozone	no	catalytic ozone converters in the supply air path

The filters only help, though, when the system is running. At the gate with the APU off and no external air conditioning supply, the air in the aircraft just sits there – which is why a full cabin during boarding on hot days often feels stuffier than in cruise. Also important: the filters protect against particles circulating through the recirculated air. Against droplets exhaled by your direct seatmate, even the best filter technology helps only to a limited extent.

Why the air on a plane is so dry

The dryness on board has nothing to do with poor technology, but with physics. At ten to twelve kilometers of altitude, temperatures are around minus 50 degrees Celsius and below. Air that cold can absorb only very little water vapor. When it is warmed to around 22 degrees, the absolute amount of water stays the same – but relative humidity falls to values otherwise known from deserts. The packs also remove moisture so that no water condenses out in the ducts.

For the cabin, the CDC quotes typical values of 10 to 20 percent relative humidity. The moisture that is present at all comes to a large extent from the passengers themselves: through breath and skin. A full cabin is therefore measurably more humid than a half-empty flight.

Why not just humidify? On classic aluminum fuselages, that is tricky: humid cabin air condenses on the ice-cold outer skin, collects in the insulation and promotes corrosion. Additional humidifiers do exist, mainly in cockpits or premium cabins of some types, but they are more the exception.

The exception: the Boeing 787 without bleed air

With the Dreamliner, Boeing took a different route. The Boeing 787-8, 787-9 and 787-10 work with a so-called no-bleed architecture: the engines supply no bleed air for the cabin and wing de-icing. Instead, each engine drives two generators, and electrically driven compressors draw in outside air through dedicated inlets ahead of the wings and bring it to cabin pressure. Wing de-icing also works electrically.

The advantages cited for this design:

• Efficiency: The air is compressed only as much as the cabin needs – instead of first producing high-pressure air and then throttling it back down.
• Weight: Long, hot pipes, valves and precoolers in the wing and pylon are eliminated.
• No oil in the air path: Since no air is routed through the engines, this possible contamination path is eliminated.
• Fewer hot lines: The hot air stays in the engine nacelle, which reduces the risk of leaks in the wing and fuselage.

The packs themselves and the HEPA-filtered recirculated air also exist in the 787. The difference lies in the source of the fresh air.

Carbon fiber makes more humidity possible

Besides the air source, the material plays a role. The fuselage of the 787 and of the Airbus A350-900 and A350-1000 from Airbus is made mostly of carbon-fiber-reinforced plastic, which does not corrode like aluminum. Both types can therefore fly on long-haul routes with somewhat higher humidity – values around 16 percent are quoted – and have a lower maximum cabin altitude than older jets. That sounds like little, but on a twelve-hour flight, many passengers notice the difference anyway. Especially on ultra-long-haul flights, such comfort factors play a big role.

Important: despite its carbon-fiber fuselage, the A350 uses bleed air in the classic way. The no-bleed solution is still a special feature of the 787 among airliners. How to tell the two widebody jets apart at the fence is shown in the guide A350 or Boeing 787?, and you can find the comparison with the Boeing 777-300ER under Telling the Boeing 777 and 787 apart.

Feature	Classic jet (e.g. A320, 777)	Airbus A350	Boeing 787
Fresh air source	bleed air from engine/APU	bleed air from engine/APU	electric compressors, dedicated inlets
Fuselage material	mostly aluminum	mostly carbon fiber	mostly carbon fiber
Recirculated air with HEPA filter	yes	yes	yes
Humidity on long-haul	low, usually 10–20 %	slightly higher possible	slightly higher possible
Maximum cabin altitude	up to 8,000 ft	lower	lower

Fume events: rare, but real

A topic that keeps coming up in the media is fume events. In these, vapors from engine oil or hydraulic fluid get into the bleed air and thus into the cabin or cockpit. The cause is usually leaking bearing seals in an engine or the APU. The crew often notices it by a musty smell reminiscent of “dirty socks,” in rare cases by visible smoke.

There are different estimates of the frequency. An often-cited estimate from US media reports is about one event per 5,000 flights; other figures are lower. Tricresyl phosphate (TCP) from turbine oils is discussed, among other things, as a possibly problematic component.

The question of long-term effects is scientifically disputed. An “aerotoxic syndrome” is not recognized medically as a distinct diagnosis; several expert panels concluded that the data are not sufficient for a robust connection. At the same time, there are studies and reports from crew members describing symptoms after such events. Authorities and manufacturers continue to work on measurement methods, sensors and maintenance standards. For you as a passenger, the rule is: a fume event is a rare maintenance and safety issue, not an everyday burden on cabin air.

Myths about cabin air

• “The airline turns down the air to save fuel.” Fresh air supply follows minimum values from the design rules. Some types allow operating one pack at reduced flow with a light load, but the required amount of fresh air per person is maintained.
• “On a plane, you only breathe recirculated air.” About half is fresh outside air, and all of the air is replaced several times an hour – significantly more often than in most office buildings.
• “The air comes from the exhaust.” Bleed air is taken upstream of the combustor and has no contact with combustion gases in normal operation.
• “You get tired on a plane because of the air.” The main reason is usually the lower oxygen partial pressure at a cabin altitude of up to about 2,400 meters, combined with dryness, sitting and time zone changes. Air quality in the sense of pollutants plays hardly any role.
• “The nozzle above your seat does nothing.” The personal air nozzle delivers the same conditioned air. A targeted airflow can help mix the air directly around you.

What you see from the outside of the aircraft

For spotters, air conditioning is not just theory. A few things you can observe on the apron or at the fence:

• Water vapor at the gate: On humid, warm days, you sometimes see mist streaming out of the cabin outlets when the packs blow in cold air. That is condensed humidity, not smoke.
• APU exhaust at the tail: When the auxiliary power unit runs, the air shimmers behind the tail cone. It supplies the packs with bleed air on the ground. How to recognize engines and auxiliary power units is described in the guide Recognizing engines.
• PCA hoses: Thick yellow or gray hoses leading under the fuselage at the gate bring preconditioned air from outside. This way, the APU can stay off.
• 787 air inlets: On the Dreamliner, the air inlets on the fuselage ahead of the wing roots, which supply the electric compressors, stand out.

Conclusion

Cabin air is one of the most tightly regulated systems on an aircraft. On almost all jets, fresh outside air comes as bleed air from the compressor, is cooled and dehumidified in the packs, mixed with HEPA-filtered recirculated air and completely exchanged several times an hour. The biggest weakness is not cleanliness but dryness – a consequence of the icy outside air at cruising altitude. The Boeing 787 goes its own way with electric compressors, and carbon-fiber jets such as the A350 and 787 fly somewhat more humid. Fume events are rare, but a maintenance topic that is taken seriously. If you understand the system, you will look at packs, APU and air hoses with different eyes on your next flight or on the apron.