From National Center for Biotechnology Information. This is one paragraph from the report at Exposure to tri-o-cresyl phosphate detected in jet airplane passengers

Jet airplane travel is safe for infrequent flyers

Half of the blood samples from the jet airplane travelers that we tested had detectable levels of phosphorylated butyrylcholinesterase, which means these passengers were exposed to tri-ortho-cresyl phosphate. The adduct levels were very low and no toxic symptoms were observed for any individual in our study group. None of our travelers reported a “fume event” in the airplane where release of engine oil into the bleed air could be detected. We were able to obtain a second blood sample from four individuals who tested positive, 3 to 7 months after their flights. The second samples were negative for phosphorylated butyrylcholinesterase. It follows that jet airplane travel is generally safe for the infrequent flyer. However, we suggest that pilots and aircrew are frequently exposed to small doses of tri-o-cresyl phosphate when they fly. Turnover of butyrylcholinesterase in the blood has a half-life of about 12 days. It follows that phosphorylated butyrylcholinesterase would accumulate in their blood. Adducts of any other proteins/enzymes that react with CBDP would likewise accumulate. Such accumulations would be compounded if individuals were to experience a “fume event” where they would be exposed to a large dose of tri-o-cresyl phosphate. The US commercial fleet is estimated to have 0.86 “fume events” per day (Murawski and Supplee, 2008). It is expected that pilots and aircrew will have significantly higher levels of butyrylcholinesterase adducts than the passengers in our study group.