Almost all passenger jet aircraft today use potentially contaminated bleed air for cabin ventilation. A detailed look at the design of engine bearings, their lubrication and sealing reveals that jet engines leak small amounts of oil by design and not only in failure cases. An equation is derived to calculate the concentration of a possible cabin air contamination. Results show good agreement with measured concentrations of hydrocarbons in aircraft cabins under normal conditions. The solution to the problem of cabin air contamination is a bleed free design. Other partial remedies are briefly discussed.