Health Concerns
The bleed air comes from the engines but is "bled" from the engine upstream of the combustor. Air cannot flow backwards though 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 components (carbon seals) can leak oil (containing highly toxic additives) into the bleed air, in what is known in the industry as a "fume event". This is generally dealt with quickly since failed oil seals will reduce the engine life.
Nevertheless, oil contamination from this and other sources within the engine bay is leading to serious health concerns, and has triggered research by a number of academic institutions.
Read more about this topic: Environmental Control System (aircraft)
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