Almost all passenger jet aircraft today use potentially contaminated bleed air for cabin ventilation. This caused an emergency landing and evacuation of Hawaiian Airlines, A321neo with smoke on board on 2019-08-22. The fume event started at top of descent, where air pressure is not sufficient to hold oil back. A detailed look at the design of engine bearings, their lubrication and sealing reveals that jet engines leak small amounts of oil by design and not only in failure cases. Oil traces can be found in bleed air and air conditioning ducts, in recirculation filters, and on cabin surfaces. The ducts are buried in the aircraft and cannot be cleaned after such a Cabin Air Contamination Event (CACE). Certification rules CS-25.1309 may not be used to argue allowance of CACEs depending on probability, because the problem is well known and needs to be rectified. Engines are today much longer on the wing without shop visit and seal replacement. Labyrinth seal clearances naturally increase as an engine ages, hence leakage increases. In addition to chemicals also metal nano particles from the engine oil find their way into the human body.
A Tue, study studied this question.