This paper describes systems commonly employed to deliver aircraft cabin air, and the internal air system which is responsible for the supply of balancing, sealing and cooling air used with the engine to control the operation of bearings, discs and other critical components, as well as bleed air. A series of mechanisms that could be responsible for contamination of bleed and cabin air are considered including: ingestion into the compressor intake of poor quality air; leakage from a seal of oil; ingestion from the gases associated with a stall or surge event; off-gassing of cabin fittings and emissions from occupants.This paper is part of: "2017 International Aircraft Cabin Air Conference : Conference Proceedings". In: Journal of Health and ...