Contamination by Bleed Air

The Bleed Air system supplies the air conditioning and pressurisation system of an aircraft with air. This air is bled from the front part of the engines, the compressors. The compressor bearing seals however, will always allow some oil leakage, even in a perfectly healthy engine! This will occur especially during power transients (e.g. during take off, climb, descent). As a result, smoke and burnt/pyrolised engine oil vapours can prevail in the air conditioning system in cabin and cockpit. During a so called Fume Event, much higher levels of contamination occur.

This contaminated air subsequently enters the cabin without any filtering. Although airlines often point at the installed HEPA filters, until today those filter only the recirculated air, air that is reused after passing through the cabin. On top of that, the vast majority of these HEPA filters are still lacking an activated Carbon filter, rendering them unfit to filter out any of the present oil particles (Volatile Organic Compounds). The air in aircraft cabins are therefore a serious threat to the health of both crew and passengers.


Aircrew regularly display a variety of symptoms of ill health. Especially after fume events, when smell (“wet dog” or “dirty socks”) and smoke appear in the aircraft cabin due to e.g. oil leakage into the Bleed Air system. The acute symptoms occurring during or immediately after a fume event, are very similar to those after exposure to carbon monoxide, formaldehyde and similar substances. The same symptoms also occur after so called Long Term Low Level Exposure.

Organo Phosphates

The Organophosphate Tricresylphosphate (TCP) and its isomers, which are present in burnt oil, cause chronic problems for the central nervous system in the human body. Besides these TCPs, many other toxic substances are formed during pyrolysis of jet engine oils. This can cause serious health problems, reflected in particular in motor skills, memory and coordination.