Why contaminated cabin air?
Even in new, well maintained aircraft, the cabin air is always contaminated (beit only light). However, this is all about a considerable number of very harmful substances, such as Organphosphates. Below is a list of the main sources of this contamination. In particular the so-called "VOCs" (volatile organic compounds) can easily settle down as a greasy deposit. That causes both the airconditioning ducts and the cabin interior to get contaminated with harmful substances. The odds of a Fume Event will rise accordingly.
What causes a Fume Event?
"Fume Event" is the term used for incidents where a serious cabin air contamination occurs. During a Fume Event, much larger quantities of harmful substances will suddenly be released. This is often (but not always!) accompanied by smoke/mist in the cabin and/or a bad smell. The smell is often described as "dirty socks" or "wet dog".
Main causes of cabin air contamination
In all present jet aircraft (except B787 Dreamliner), cabin air is supplied by the so-called Bleed Air system. This system allows numerous harmful substances to easily enter the cabin, such as:
The oil used in jet engines contains several chemical additives, including 3-5% of Organophosphates like Trihresylphosphate (TCP). These substances are among the most toxic compounds known to man. Inhaling Organo Phosphates is very detrimental to health.
The hydraulic fluid used also contains several highly toxic components, including TBP (Tributylphosphate). And these substances will also end up in the cabin, e.g. because of leakages or overfilling.
Bleed Air is heated up to around 400º C in the compressor. Incomplete burning of the oil particles present will occur, and the formation of Carbon monoxide is likely to occur as well.
2) De-icing Fluids
De-icing fluids are used during winter, to clear the aircraft of snow and ice. This is necessary in order to prevent performance degradation. When the fluid is sprayed too close to the APU air inlet or the engines, they will also enter the cabin via the airconditioning.
Some countries demand airlines to spray the aircraft cabin with insecticides. Sometimes this is done before the passengers embark, but sometimes it is done during the flight. These toxins will once again contribute considerably to a further contamination of the cabin air and interior.
The so-called Auxiliary Power Unit (APU) is an extra turbine engine in the back of the aircraft. It delivers electrical power to the on board systems, and bleed air for the air conditioning systems while the engines are not running. The APU is a regular source of cabin air contamination. This is caused by e.g. engine oil or hydraulic fluid leaks, or e.g. after an oil overfill.