At high altitudes where commercial jetliners operate, bleed air is used to supply crew and passengers with fresh air at the right temperature. This bleed air is taken from the engine and mixed with recirculated cabin air. The bleed air is compressed to cabin pressure by the engine compressor, which is lubricated with synthetic oils, containing poisoning components. If the seals used are not a 100%, crew and passengers are at a risk of inhaling toxic air, leading to aerotoxic syndrome.
Other than the 787, all commercial aircraft, even the A350 (and most probably 777X) use bleed air for the cabin air supply. This is due to the fact that the engine compressor is overall the most efficient air compressor on-board jetliners. Besides, an additional compressor adds more weight, which translates in less payload and/or range. In recent years, most efficiency improvements on aircraft have come from engines and so the compressor became more efficient too. An additional compressor with clean air for the cabin consumes electrical energy. This energy has to be generated by the engines and transported to the compressor. So the case for using the engine-based compressor is more compelling.
That said, we see how safety and operational efficiency might impair each other; airlines and OEMs have to decide if and how to make bleed air cleaner. The UK Committee on Toxicity of Chemicals in Food, Consumer Products and the Environment (COT) accepts that fume events occur on one flight in 100 according to the Aerotoxic Association. A documentary about aerotoxic syndrome has been released by Tim van Beveren, an aviation journalist.
We are curious how the industry perceives and solve the problem with the new generation of aircraft coming on stream. Moreover we hope that the industry finds a satisfactory answer for the current generation of aircraft.