We present strong evidence for the presence of aerosols of Nano-particles (also termed Ultrafine Particles (UFPs) in aerosol science) in the breathing air of commercial aircraft using engine bleed air architecture. The physical and chemical nature of engine oils and the high temperatures attained in aircraft jet engines (up to 1700°C in the oil circulation and up to 30,000°C in the bearings) explain why UFPs are to be expected. A discussion of oil seals used in gas turbine engines concludes that they will permit UFPs to cross them and enter the breathing air supply, in conjunction with a complex mixture of chemicals such triaryl phosphates which are neurotoxic. A consideration of the toxicology of Nano-particles concludes that their continual presence over a typical working lifetime of up to 20,000 hours in aircrew will predispose them to chronic respiratory problems and will exacerbate the translocation of neurotoxic substances across the blood brain barrier
