# How does air enter and leave the cabin of an airliner? [duplicate]

Yesterday I flew on Airbus A320 and on the airline's board catalogue, there were written some "curiosities". On was written that the air coming from the air conditioning inside the fuselage is very dry, (3-10% humidity) because while it's warmed up from the freezing outside air, it is much more complicated to humidify it.

Firstly I never even thought that our breathable air comes from the outside.

Questions:

• During flight, from where air intake is made in an aircraft?
• Does this air also contain CO2 along with Oxygen dumped directly to the fuselage?
• As an aside it is much cheaper to heat low humidity air. The air is just coming from the outside (indirectly through the engine which provides high pressure air), so there is no need to modify the $O_2$, $CO_2$ content. – copper.hat Feb 9 '15 at 16:01
• @Th3alchemist the air always contain some amount of CO2 and larger amount of O2 wherever you go. It is outside air, not from special tank. – vasin1987 Feb 9 '15 at 16:21
• Ok thanks for clarifying that. Sorry the duplicate post. – Th3Alchemist Feb 9 '15 at 22:18