A respected aviation safety expert has told ITV News that the government has misled the public regarding the risk of contaminated cabin air on planes.
Flying is an act of faith and record numbers of us do, but few of us realise that the air we breathe on-board comes from the engine - and that potentially dangerous chemicals can, and do, get inside cabin.
ITV News has spoken to a cabin crew member who says he was diagnosed with carbon monoxide poisoning after a flight he was on three years ago. He still works for the airline so we agreed to conceal his identity
About an hour into the flight some of the crew started to talk about a smell. I started to feel quite unwell myself.
The airline paid him compensation. He believes it was oil fumes from the engine that made him ill.
Most jet engine lubricating oils contain organophosphates which can be toxic but are added to the oil to prevent parts of the engine from wearing out and over-heating.
Engines operate at high temperatures and when heated jet engine oil can give off fumes which are harmful and irritating, including carbon monoxide.
Manufacturers of jet engine oil warn that breathing in those fumes can causes headaches, nausea and soreness in the eyes, nose and throat.
There has been plenty of scientific research into the quality of cabin air: Cranfield University published the result of 100 in-flight tests in 2010; The Institute of Occupational Medicine published the results of swab samples in 2012; the evidence was reviewed in 2013 by the independent Committee on Toxicity, which advises the government.
The regulator, the Civil Aviation Authority, says the conclusions of all the scientific studies is that there was "no positive evidence of a link between exposure to contaminants in cabin air and possible acute and long-term health effects, although such a link cannot be excluded."
Frank Taylor is a member of the UK Air Safety Group, a voluntary organisation of aviation professionals including engineers, pilots, cabin crew and air traffic controllers.
The group was set up in 1964 to highlight safety concerns it felt the authorities weren't paying enough attention to.
Mr Taylor told me that the results of the Cranfield study have been "totally misrepresented by the Department for Transport". He says the study failed to capture what happens during a "fume event" and therefore more research is needed.
The government's position is that contaminated air doesn't pose a risk to health. Mr Taylor disagrees.
"I think that (position) is naive," he told ITV News. "I think it does pose a risk to the health to a small number people, not to everybody. And if those people are pilots then we have a problem. If two pilots are affected on the same flight, then we have a major problem".
Last month ITV News revealed that between December 2014 and March of this year, smoke or fumes were reported in the cabin on 167 flights by British registered airlines.
On 15 occasions pilots were so concerned by the fumes they made emergency calls to Air Traffic Control.
Pilot Richard Westgate was on medical leave when died in 2012. He believed he'd been poisoned by repeated exposure to contaminated air.
In February, the coroner leading his inquest called for "urgent action to prevent future deaths."
To date, the government hasn't acted.
The aviation minister, Robert Goodwill MP, told me:
The science at the moment does not indicate further research is necessary but we'll keep an open mind.
The minister seems confident, but Richard Westgate's coroner has concerns. The inquest is expected to conclude towards the end of this year.