I understand that the problem is fumes from the burnt engine oil, getting into the engine bleed air and thus into the aircraft pressurisation system. For some reason the lubricating oil contains organophosphates and these are a major risk to health. See link re problem with this chemical and the health of vets who have to supervise the dipping of sheep with a 'dip' which also contains organophosphates.
http://news.bbc.co.uk/2/hi/health/572960.stm