I also believe the problem came to a head with the Bae146 which was powered by Four ALF502 engines.
My understanding was that the contaminated air came from a dodgy seal on the gearbox? of the APU on the 146s. Bad/old/damaged seals allowed oil to pass through to and become heated (not burnt per se) so that the toxic component (tricresyl...) entered the bleed air.
There was a case on this a few years ago that was brought by a member of the cabin crew before the Dust Diseases Tribunal. If I remember rightly I think she was successful.
Scary stuff.