Editing Environmental control system (section)

==Health concerns==
{{main|Aerotoxic Syndrome}}

The bleed air comes from the engines but is bled from the engine upstream of the combustor. Air cannot flow backwards through the engine except during a [[compressor stall]] (essentially a jet engine backfire), thus the bleed air should be free of combustion contaminants from the normal running of the aircraft's own engines.

However, on occasions carbon seals can leak oil (containing potentially hazardous chemicals) into the bleed air, in what is known in the industry as a [[fume event]].<ref>{{cite news|author=The Guardian|title=Toxic cockpit fumes that bring danger to the skies |url=https://www.theguardian.com/airlines/story/0,,1718316,00.html|access-date=2007-10-20 | location=London | date=2006-02-26}}</ref> This is generally dealt with quickly since failed oil seals will reduce the engine life.

Oil contamination from this and other sources within the engine bay has led to health concerns from some advocacy groups and has triggered research by several academic institutions and regulatory agencies. However, no credible research has yielded evidence for the existence of a medical condition caused by fume events.<ref>{{cite web|url=http://www.esam.aero/main/docs/ecam08/No%207%20Bagshaw%20paper.pdf |title=The Aerotoxic Syndrome |publisher=European Society of Aerospace Medicine |date=September 2008 |access-date=December 31, 2012 |author=Bagshaw, Michael |url-status=dead |archive-url=https://web.archive.org/web/20120227182208/http://www.esam.aero/main/docs/ecam08/No%207%20Bagshaw%20paper.pdf |archive-date=February 27, 2012 }}</ref><ref>{{cite report|chapter-url=https://publications.parliament.uk/pa/ld199900/ldselect/ldsctech/121/12107.htm|author=Select Committee on Science and Technology|publisher=House of Lords|title=Science and Technology - Fifth Report|chapter=Chapter 4: Elements Of Healthy Cabin Air|access-date=2010-07-05|year=2000|archive-url=https://web.archive.org/web/20100424020804/http://www.publications.parliament.uk/pa/ld199900/ldselect/ldsctech/121/12107.htm|archive-date=2010-04-24|url-status=live}}</ref><ref>"Aircraft fumes: The secret life of BAe", "In the back" column, Private Eye magazine, issue 1193, 14–27 September 2007, pages 26–27; Pressdram Ltd., London.</ref>