A fume event occurs when bleed air used for cabin pressurization and air conditioning in a pressurized aircraft is contaminated by chemicals such as engine oil, hydraulic fluid, anti-icing fluid and other potentially hazardous chemicals including cargo spills. Turbine engine oil is an irritant and contains neurotoxic chemicals such as tricresyl phosphate. However, no conclusive evidence exists that engine oil contains sufficient quantities of such chemicals to cause long term damage. Hydraulic fluid is extremely irritating, particularly to the eyes, which creates a hazard to pilots during a fume event. However, it is non-toxic and does not cause lasting damage. Anti-icing 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 for the air to be breathable. This is done by drawing pressurized air from the engine after it is compressed, but before it is exposed to fuel or oil. This air is known as bleed air. After the hot, pressurized air is drawn from the engine, it is cycled through air conditioning packs where it is cooled, humidified and sent into the cabin.
Handling of fume events 
In the event of fumes or smoke in an aircraft, the crew will wear oxygen masks in order to avoid breathing in irritating fumes. Goggles are also available if necessary. If the fumes do not subside after an attempt is made to diagnose and fix the problem, the flight is then diverted to a nearby airport. In a severe fume or smoke event, the flight may descend to an altitude of 10,000 feet or less where it can safely be depressurized
Health effects 
The potential long term health effects of fume events are currently disputed. Most aviation and medical professionals do not believe any long-term health effects exist from fume events, and no studies have shown any danger to the public. However, some consumer and aircrew advocacy groups claim that it can cause a medically unrecognized condition they refer to as aerotoxic syndrome, with symptoms such as fatigue and nausea.
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