The air that you breathe in an aircraft cabin is known as bleed air, drawn from the compressor stage of a jet engine.
While this system provides a clean and pure air supply for the cabin, it can happen that damaged seals allow lubricating oil to leak into the compressor, and into cabin air.
This oil contains organophosphate compounds, toxic substances that can cause blurred vision, nausea, breathing difficulties, headache, fatigue, convulsions and even death.
Exposure to this contaminant in the aviation environment has been labelled aerotoxic syndrome, and the effects can be long-term and life-threatening.
Over several decades a disproportionate number of cabin crew, pilots and frequent flyers have reported symptoms of aerotoxic syndrome, tagging it the "asbestos of the skies".
The aviation industry and its regulators have consistently denied the existence of aerotoxicity, yet in 2014, a post-mortem showed that a British Airways cabin crew member who died suddenly after a six-hour flight was found to have high levels of organophosphate poisoning, one of the effects of aerotoxic syndrome.
One airline is finally tackling the issue. In 2017 British carrier easyJet began trialling a new air filtration system, a move regarded as an acknowledgment of aerotoxic syndrome.