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.

A minor degree of 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. Many of the likely substances involved in cabin air contamination events are also general environmental pollutants and so persons who do not fly may be exposed to these substances to a lesser or to a greater degree. Hence, although these contaminants might be measured and found to be at a particular level in aircraft, they may be found at higher levels in homes, offices and other modes of transport.

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.

 BALPA Position

In the event of contamination of the cabin air, we can foresee the possibility that harm could be caused to aircraft occupants who have no choice but to breathe the supplied air. We also consider that this harm could, in principle, be mitigated by the installation of bleed air filters or detection devices that can monitor the bleed air for contamination. In practice, however, neither of these technologies are fully proven and so we have asked EASA to commission further study into these technologies.

The basis for our position is only for the reasons stated above, it is not because we advocate the existence of "aerotoxic syndrome", a proposed long-term health effect attributable to breathing cabin air. This point of clarification is important because “aerotoxic syndrome” is reportedly characterised by a wide range of symptoms that are very common in the general population, including those who have not flown.

Our principal instrument for the investigation of health effects is the cabin air care pathway.