A fume event occurs when bleed air used for cabin pressurization and air conditioning in a pressurized aircraft is contaminated by fluids such as engine oil, hydraulic fluid, anti-icing fluid, and other potentially hazardous chemicals. Turbine engine oil is an irritant and contains neurotoxic chemicals such as tricresyl phosphate. The aviation industry claim that engine oil does not contain sufficient quantities of such chemicals to cause long-term damage. However, there is some historical evidence that would seem to contradict this statement. In 1959, over 10,000 people in Morocco were paralyzed or otherwise affected after ingesting small quantities of tricresyl phosphate in their cooking oil. Unlike tricresyl phosphate, hydraulic fluid — although non-toxic in small quantities — is extremely irritating to the eyes and skin, which creates a hazard to pilots during a fume event but causes no lasting damage. Deicing fluid has a strong smell but is not very irritating or toxic.
How cabin pressurization works
Because airliners fly at very high altitudes, the cabin must be pressurized to provide a safe quantity of breathable oxygen to passengers and crew. The cabin is pressurized with bleed air tapped from the jet engine's compressor sections, which are prior to the combustion sections. That air is very hot and must be cooled by heat exchangers before it is directed into the air conditioning units, which cool it even further.
Handling of fume events
In the event of fumes or smoke in an aircraft, flight deck crew will wear pressurised oxygen masks in order to avoid breathing in irritating fumes. Goggles are also available if necessary. However, no protection is available for passengers, as cabin oxygen masks are not intended to protect against fumes, and will not drop unless a depressurisation occurs. Cabin crew may be able to use portable oxygen masks if they identify the fume event in time. If the fumes do not subside after an attempt is made to diagnose and fix the problem, the flight is diverted to a nearby airport. In a severe fume or smoke event, the aircraft might descend to an altitude of 10,000 feet (3,000 m) or lower where it can safely be depressurized. 
The potential long term health effects of fume events are disputed and sometimes confused with other common ailments as there can be a delay between initial exposure and onset of symptoms. Most aviation and medical professionals do not believe any long-term health effects exist from fume events. However, some consumer and aircrew advocacy groups claim that it can cause a medically unrecognized condition they call aerotoxic syndrome, with symptoms such as fatigue, nausea, and possibly premature death. In 2015 a captain of an airline was incapacitated in flight by a fume event. His heart failure and death 50 days later was ruled to be directly connected to the exposure to the fume event. 
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- "Aerotoxic.org". Retrieved December 31, 2012.