Professional articles by ASHRAE www.ashrae.org, 2017 Winter Conference-Papers
The Nature of Particulates in Aircraft Bleed Air Resulting from Oil Contamination
The vast majority of commercial aircraft utilize bleed air from the propulsion engine compressors for ventilation and pressurization during flight. They may also use an auxiliary power unit for this purpose during ground operations and some phases of flight.
If there is contamination from lubricating oil or other fluids ingested into the engine air intake or through bearing seal leakage, then the air supplied to the aircraft will also be contaminated.