LONDON — For years, airline crews have been reporting ‘fume events’ on board aircraft. Some have linked the presence of toxic substances in cabin air to ill-health, even deaths.

Now, a UK trade union is actively seeking evidence to take airlines to court over the issue.

The UNITE union represents around 20,000 cabin crew in the UK airline industry and is “now in the early stages of pulling together relevant evidence,” said spokesman Alex Flynn. When, or if, sufficient is gathered, personal damages cases will be lodged in county courts, seeking damages for alleged damage to the health of the cabin crew involved.

The union is examining 17 cases, which surfaced after it set up a confidential helpline around four weeks ago. “We anticipate that number will grow,” said Flynn.

The move by the union is the latest development in a story that has rumbled away in the background for several years. Flightdeck and cabin crew have complained of health problems resulting from contamination of cabin air. There can be various causes for unpleasant smells or even visible vapours onboard, but the one causing most concern to campaigners is the ingestion of engine bleed air contaminated by products from engine oil.

Engine bleed air is used in all modern airliners with the exception of the Boeing 787. Normally, bleed air is kept separate from the engine’s oil by a system of valves and seals, but can be contaminated if these deteriorate or fail.

Such a failure can result in the air becoming tainted with chemicals within the oil, such as organophosphates. Pilots and cabin crew have complained of a variety of ailments after fume events, ranging from headaches, blurred vision and nausea to longer-term effects such as loss of memory. Collectively, these are referred to as aerotoxic syndrome.

In February, a UK coroner conducting inquiries into the 2012 death of a 43-year-old British Airways pilot, Richard Westgate, said publicly that toxic fumes in cabin air could pose a health risk. He wrote to both British Airways and the UK regulator, the Civil Aviation Authority, setting out his concerns over the presence of organophosphates in cabin air and that these could cause illness or even death.

As well as setting up its helpline, UNITE is creating a ‘fume event register’ to monitor occasions where their members believe they have been exposed to contamination, to build up a more accurate picture of the frequency of such events.

It is also supporting the family of UNITE cabin crew member Matthew Bass, who died suddenly aged 35 and whose death is the subject of an imminent inquest.  A second, specialist autopsy on his body is said to have found high levels of organophosphates.

Airlines and regulators in Europe have consistently said that no evidence exists to link fume events with dangers to health.

The UK Department for Transport (DoT) funded a study by Cranfield University from 2008-12, which looked for evidence of contaminants in 100 flights. Nothing infringing existing safety limits was found and the study concluded that “additional studies are required to characterise the air quality during the occasional events reported by crew.”

The DoT also noted an earlier study by the UK’s Independent Committee on Toxicity (a panel including toxicologists from various universities), which completed a review of fume event evidence in 2007. That found that such incidents occurred on one in every 2000 flights.  In typically noncommittal civil service fashion the DoT concluded that “the evidence available did not establish a link between cabin air and pilot ill health, but nor did it rule one out.”

Meanwhile, four flight attendants from Alaska Airlines filed a suit in the U.S. Circuit Court of Cook County, Illinois, accusing Boeing of fraud, negligence and design defects in the Boeing 737-800 aircraft, after a fume leak in a flight in 2013 caused “devastating health effects on them.” According to the suit, among long-term medical conditions are memory issues, tremors, blinding headaches, fatigue and gastrointestinal problems.

One problem in gathering evidence is that the incidents are, by their nature, transient and so it can be difficult gathering evidence of any level of toxicity that may occur.

Now, the European Aviation Safety Agency (EASA) has awarded a contract to a German research center to monitor a further 60 flights to gain more knowledge of the problem. This will look at various aircraft and engine combinations and will continue until Q3 2015.

EASA spokesman Dominique Fouda said that the subject “is being taken very seriously” but, at present, “on available knowledge, it leads us to think that there is no unsafe condition that has been identified. “

The CAA said that fume events were the subject of Mandatory Occurrence Reports and that it was supporting the EASA research initiative.