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)
Famous quotes containing the words health and/or concerns:
“As they move into sharing parenting, men often are apprentices to women because they are not yet as skilled in child care. Mothers have to be willing to teach fathers—both by stepping in and showing and by stepping back and letting them learn.”
—Nancy Press Hawley. Ourselves and Our Children, by Boston Women’s Health Book Collective, ch. 6 (1978)
“Our ideal ... must be a language as clear as glass—the person looking out of the window knows there is glass there, but he is not concerned with it; what concerns him is what comes through from the other side.”
—Elizabeth Bowen (1899–1973)