Frequently Asked Questions

Why Do Airlines Use "Recirculated" Air? Will it make me sick?

One writer says he gets sick when the engines are started:

“…the air in the cabin kicks on and sends very smelly jet exhaust into the cabin.  I would think that the air would be controlled by the pilots or stewardesses on the airline.  The air really doesn't need to be [turned] on until…take off.  I don't know if it's a cabin pressure situation or not, but the gasses that enter the cabin make me very sick for days after the flight….”

Another says: “I need to fly on a commercial carrier, but I cannot afford another respiratory disease. I fear the air quality onboard.”

Another wants to know if recycling the air reduces the "air quality" enough to make it dangerous to her infant child.

See also, a Letter, proposing a solution.

Editor's Reply:

It is possible to smell exhaust fumes, from the plane taxiing ahead of you.  Pilots try hard to avoid that situation, but airport congestion, combined with contrary wind conditions, sometimes makes that goal difficult to achieve.  It isn’t possible to smell the exhaust fumes from your own plane as the intake of the air-conditioning “packs” is far upstream of the engine exhaust cones.

Passengers radiate body heat and exhale carbon dioxide.  That makes it necessary to turn on the packs (air conditioning machines, driven by jet engine bleed air), while the plane is full of passengers on the ground.  It wouldn't take long for both temperature and carbon dioxide to reach unacceptable levels if new air wasn't brought into the cabin, exhausting the old in the process.  Additional fans are also turned on, to facilitate the flow of air from those packs.

In flight, the packs are usually operated in “high flow” mode during ground operations, and the climb/descent portions of the flight.  Recirculation fans are also running to enhance the velocity of the air in the cabin.  When cruise altitude is reached, it is normal for the packs to change from high flow to “normal” flow.  That means they pump air into the cabin at a somewhat lower volume (cubic feet per minute) than they do in the high flow mode.  That is done to reduce wear and tear on the packs and to save fuel.  The laws of physics require additional energy consumption to pump higher volumes of air.  That requirement is even more pronounced with the newer energy efficient and environmental friendly engines of today:

"... most newer jetliners are powered by high-bypass-ratio fan engines which are much quieter, much cleaner burning, more powerful and much more efficient. At the front end of this engine type is a large-diameter fan, which is powered by the core. The fan moves a large volume of air past the core rather than through it, and actually generates most of the thrust. Every unit of pressurized air extracted from the engine core has the effect of reducing fan thrust by an even greater amount, and that degrades fuel efficiency more severely on this type of engine than on the older type. By providing the cabin with a mixture of about 50 percent outside air taken from the compressor and 50 percent recirculated air, a balance has been achieved that maintains a high level of cabin air quality, good fuel efficiency and less impact to our environment."  [Excerpt from Boeing consumer news.  For the entire document, and more, click here: Boeing. See also, Facts and Myths]

Some airliners have additional recirculation fans which turn on when the packs switch to the normal mode.  Recirculation fans are likely what the so-called "consumer activists" like, Ralph Nader and Jeremy Rifkin, are referring to when they claim airlines are more concerned with profit than supplying healthful air to their passengers.  As always, when capitalist haters like Nader, Rifkin, et. al., are grinding their agenda axes, it is important to look for the information they leave out:  Quite deliberately, they avoid the fact that despite the use of recirculation fans, in-flight cabin air is of better quality than air in the average office building and is changed completely in a much shorter period of time (3 to 5 minutes).  It simply is not necessary to remain in the high flow mode because the air, at cruise altitudes, is much cooler and far less polluted than the air we breath at ground level. And, shutting off the recirculation fans, which would limit air flow to 100% outside air, would lower the humidity too much. That would cause health problems.  

Exposure of an infant to a crowd of people carries pretty much the same risk, regardless of where that crowd of people is located. The flu virus becomes airborne and is spread when someone coughs or sneezes.  In a no-wind situation, it is quite possible for someone 50 ft. away to become infected by that airborne virus.  There is always some risk of being exposed to infections from others, whenever you leave your own house.  It is well known, that children who are daily placed in “child care” facilities suffer much greater exposure to contagious diseases than those who are cared for in their own home.  Exposure to additional humans, anywhere, increases the risk of contracting the diseases others may harbor. 

If you are unwilling to fly because of that risk, then you also shouldn't go into office buildings, shopping centers or any other kind of enclosure that has both a managed air environment and lots of other people.  Air in those structures isn’t completely changed as rapidly as the cabin air of the typical airliner.  I know of no credible evidence to show that the use of airliner recirculation fans increases the risk of contracting contagious diseases. For a detailed discussion on cabin air quality see the Boeing Forum Newsletter.  See also, the Letter that raises more questions on this subject.  And, more information from 

July, 2000, Revised February, 2001

Robert J. Boser


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The Editor of this Web Page, now retired, was an airline pilot for 33 years and holds 6 specific Captain's type-ratings on Boeing Jet Airliners.


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