Aircraft that source the cabin air supply from the compressor stage of turbine engines are documented to become contaminated with engine oils and other aircraft fluids. This can occur at low levels on a frequent basis during normal engine operation or occasionally at higher levels during abnormal operating conditions. As there are no detection systems on board aircraft to identify this unfiltered contaminated air, ad hoc studies have been undertaken looking for individual substances above set limits. More recently there has been increasing interest in Ultrafine Particles (UFP) that have been associated with engine oil leakage into the air supply during normal engine operation. A small scale proof of concept study was undertaken on 4 short haul aircraft to assess whether UFPs are elevated during various phases of engine operation when oil leakage into the air supply is expected to occur. UFPs were shown to be elevated up to near 100,000 particles/cm3 during engine power changes or air supply configuration changes. These are the times when it is expected that the background continual very low level leakage of oil past the engine oil seals will increase, of which some is expected to enter the air supply. Engine generated oil leakage generates UFPs during normal engine operation.