ONLY a catastrophic passenger plane crash will force the aviation industry to come to terms with its "dirtiest secret", according to the director of an explosive new documentary on toxic cabin air.

German investigative journalist former pilot, Tim van Beveren, said airlines and regulators would continue to turn a blind eye to cases of pilots and cabin crew being sickened or incapacitated by poisonous fumes leaking from the engines until there was a "tombstone event" - or fatal crash.

Mr van Beveren was speaking at the Grosvenor cinema in Glasgow, where his film on the controversial issue of aerotoxic syndrome will receive its UK premiere tomorrow [Friday].

He said: "They call it a 'tombstone event', or 'tombstone imperative', where a cost-benefit analysis is applied to each and every safety intervention. It basically means there needs to be a body count before there is a change.

"This is so contradictory to everything I learned from my first flying lesson where absolutely everything is done to prevent an accident, but now all of a sudden we these areas where there's a huge gap and no one wants to take responsibility and do anything about it."

The film, 'Unfiltered Breathed In: the Truth about Aerotoxic Syndrome', accuses airlines, manufacturers, and regulators worldwide of helping to cover up a health scandal that would cost the industry billions in aircraft modifications and compensation if admitted.

The controversy relates to the use of "bleed-air" to pressurise and ventilate aircraft cabins and cockpit, which has been standard design on passenger jets since the 1950s. All commercial planes - with exception of Boeing's new Dreamliner - use the bleed-air system, which means the air breathed by passengers and crew is contaminated by low-level concentrations of poisonous chemicals coming from the engines.

These compounds, known as organophosphates, have been blamed for causing various neurological symptoms such as numbness, memory loss, mobility problems, and headaches in thousands of pilots and cabin crew worldwide, though industry insists there is no evidence of any connection.

In some cases, severe fume events in the cockpit have left pilots compromised at the controls. The film recounts one case in Germany in 2010 saw both the captain and co-pilot of a Germanwings plane overcome by fumes on approach to Cologne which left them nauseous, dizzy, suffering from tunnel vision, concentration problems and severe tingling.

By the time they put on oxygen masks and recovered some control, the plane was travelling too fast for an autopilot landing and the captain - despite feeling unwell - had to land the aircraft manually. Both pilots, who described feeling in a "dream-like" state during the landing, were subsequently signed off sick for six months.

Mr van Beveren believes the incident was a "near-miss".

The film, which draws parallels with Gulf War Syndrome, also followed the cases of several former airlines stewards suffering chronic and unexplained neurological disorders which they believe were caused by exposure to aircraft toxins.

Researchers interviewed for the film believe that around three per cent of the population are genetically susceptible to the contaminants and will go from "fit and healthy to an invalid" within three years of working on airlines.

Around 30 per cent will take up to 20 years to develop symptoms but two thirds of the population will never fall ill, leading to accusations that sufferers are "imagining it".

Richard Westgate, a former British Airways pilot who died in 2012 after years of symptoms which his parents likened to "increasingly advanced stages of multiple sclerosis", was eventually labelled with Munchausen's syndrome - where patients feign illness for attention.

However, analysis of his brain, heart, nerves and spinal cord following his death - he donated his remains to advance scientific research into aerotoxic syndrome - have revealed nerve damage consistent with organophosphate poisoning. An inquest is on hold.

Glasgow-based aviation lawyer Frank Cannon, who represented Westgate and currently has "hundreds more" cases on his books, said he recently settled one pilot's case out of court for €250,000.

Mr Cannon cannot name the airline involved, but said his client has been left unable to work due to a neurological disorder.

The film's premiere comes less than a week after joint study by the Open University and University College London stressed the need for more research.

Co-author Dr Gini Harrison said: "While the existence of a relationship between contaminated cabin air and ill-health may be a potentially expensive and inconvenient truth; the costs of ignoring the possibility of such a relationship are too high to ignore."

Mr van Beveren wants all passenger aircraft fitted with filters to prevent engine chemicals leaking into cabin air. The devices are already used on DHL freight planes.

A spokesman for UK regulator, the Civil Aviation Authority, said passenger and crew safety was "of paramount importance to the CAA".

He added: “Several expert studies on the issue of cabin air quality have been carried out in recent years including Government-commissioned research. The overall conclusion has been that there is no positive evidence of a link between exposure to contaminants in cabin air and possible long-term health effects - although such a link cannot be excluded."