In a typical commercial cabin air recirculation system, the air supplied into the cabin consists of approximately:

50% outside air from either the engine's compressor stage (engine 'bleed air') or the Auxiliary Power Unit (APU) mixed with approximately, and the other 50% of filtered, recirculated air.

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How to Know Your Plane Part 1

However, the new generation of airplane (B787) do not use bleed air..

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NAC