The BBC has been told of new research linking toxins found in the air systems of commercial airliners and neurological damage suffered by pilots.
An international collaboration between scientists suggests a direct link between the so called aero-toxic syndrome and chemicals present in cockpit and cabin air supplies.
It is estimated that fume contamination occurs on at least one in 2000 flights.
For Tony Watson, a commercial pilot for the best part of a decade with a major UK carrier, flying was his life.
But Mr Watson believes he was poisoned by the aircraft he flew. Complaining of neurological symptoms like muscle twitching and tremors as well as problems with his short-term memory, he decided to quit.
Half the cabin air onboard aircraft is siphoned in through the engine
"It came to the point when I said I'm not fit to fly anymore," he says.
"I was worried that I was becoming unsafe because I couldn't function to a good enough standard in the cockpit."
He underwent neurological tests which he says show damage to the neurological system consistent with exposure to toxins, plus blood tests which show petroleum related chemicals which you find in the oil in the engine plus nickel and cadmium which you also find in aero engines.
"I can't see any other explanation for these things," he says.
According to pilots and campaigners, the problem lies with some aircrafts' air systems.
Half of the air we breathe on board is re-cycled. But the other half is drawn through the heart of the jet engines.
This, known as bleed air, is cooled and pumped directly into the aircraft.
The problem comes when a fault occurs in the engines and they pump out a cocktail of potentially poisonous gases. And, with no air filters, these toxins end up in the cockpit and the cabin, in the air we breathe.
John Hoyte was a pilot for 30 years. He gave up his licence because of ill-health four years ago. In 2007 he set up the Aerotoxic Association.
He says that "a couple of hundred people" have contacted him over the past two years, "cabin crew mainly and pilots who've got problems".
Mr Hoyte points the finger at the air industry. In 2004, the British Airline Pilots Association highlighted the problem of fume contamination events, pointing out the main offenders as the BAe 146 and Boeing 757, popular commercial tourist and business airliners.
John Hoyte believes the airline industry is responsible for toxic cabin air
There are no accurate figures for the number of fume events - they aren't collated, but according to the independent Committee on Toxicity it's estimated that they occur on at least one in 2,000 flights.
Now one scientist has told the BBC of new evidence of a direct link between fume contamination and neurological damage in pilots.
Dr Peter Julu is a consultant Neurophysiologist at the Breakspear Clinic in Hertfordshire, the Royal London Hospital and Aalborg University in Denmark.
He has collaborated with other international research scientists, testing pilots said to be suffering from the so-called Aerotoxic Syndrome.
Researchers in the US have found toxins - known as organophosphates - in the blood and fat tissue of 26 pilots. Dr Julu has been carrying out further tests for neurological damage.
"The only connection I can derive from there is the organophosphate, meaning that the pilots who have been tested have suffered neurological damage because of organophosphates that they were exposed to while they were on the planes," he says
And, even though this research has yet to be peer reviewed, Dr Julu says that his science is safe. "With the autonomic nervous system you cannot manipulate it. What we get is objective, quantitative and standardised measurement."
Dr Julu has tested 18 pilots - half from the US study and the others independently. The tests show, he says, that they suffered chronic low level exposure to organophosphates.
Both BAe Systems and Boeing declined the offer of an interview. In a statement BAe said it has trialled a new air filtration system which is currently being fitted to aircraft.
It added: "BAe Systems regards the safety of its fleet of aircraft and those who operate them as of the utmost importance. The air quality on the BAe 146 has been shown by independent studies to exceed all existing international standards."
Boeing issued the following statement: "It is our belief that air quality on airplanes is healthy and safe. This belief is based on a number of studies that show measured contaminant levels are generally low and that health and safety standards are met."
Professor Muir's report on aerotoxic syndrome is expected soon
In 2007, following concern about the lack of definitive research into fume events, the Department for Transport commissioned Cranfield University to investigate the problem. Professor Helen Muir has been working on the study since 2008 - she expects to report her findings in six months' time.
There will be organophosphates on the flight deck, she says. But, what matters is, are they there in sufficient concentrations to potentially cause harm to people?
"My report will simply be a detailed analysis of what we find. It will be then up to the medical side to determine whether what has been found in the atmosphere has the potential to cause some of the illnesses that have been reported," she says.
For most observers the jury is still out - they await the findings of Professor Muir's research. But it's understood that if it finds that there is a problem, some carriers are already preparing to pay out compensation.
For campaigners and victims alike this is an industry wide problem risking the health not only of pilots and cabin crew but the airborne travelling public world wide.
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