This document (below) from 1955 discusses the causes of contaminated air and filters to stop this from happening. IT IS 2018 - Why has nothing been done to stop contaminated air from entering cockpits and cabins? Why is this subject continually denied and played down by governments, industry regulators and airlines? How much longer will the denial and lies go on for? How many more people have to die or become ill because of contaminated air in aircraft? Why is no one interested in the damage that contaminants from aircraft oil (heated in engines up to 700 degrees) do to the human body? It’s a crime. https://www.aerotoxic.org/pdfs/reddall-1955-elimination-of-engine1.pdf