Cabin Air Quality

A jet engine essentially works by compressing air in its front part; the compressed air then flows to the middle part of the engine where fuel is added and burnt; the expanded hot gases then blast out of the rear part of the engine.
 
At jet aircraft operating altitudes, the air pressure is too low to provide enough oxygen for the passengers and crew. In most jet aircraft, to supply air to the aircraft cabin at a pressure sufficient to allow the aircraft occupants to breathe comfortably, cabin air is tapped (bled) from the front compressor part of the jet engines. Bleed air from the engine is initially hot due to its compression and some of this heat is used to warm the cabin; it is then further cooled before entering the cabin. Typically, bleed air is not directly filtered when it first enters the air conditioning system, although it is filtered when recirculated back into the cabin.
 
Contamination of the bleed air by engine oil that has leaked past oil seals, plus hydraulic fluid, de-icing fluid, fuel, and other environmental contaminants can occur in bleed air supplied cabins. There may be many thousands of potential contaminants arising from aircraft engine oil and other fluids, but the range and quantities depend on multiple factors such as the age of the oil and the very specific conditions that an individual engine may generate.
 
To a degree, the toxicology of the contaminants, alone or in combination, is uncertain. However, it is known that some of the likely contaminants, if in sufficient quantity, are harmful, and it is also known that the toxicity of mixtures of substances may be greater than the sum of the individual substances.