Airline staff say poor air quality in planes is making them ill.
The American union which represents flight crews is calling for tighter regulation of air quality on commercial planes.
It has collected 760 examples from members of what it says are health problems caused by poor air quality.
But scientists told a US government scientific panel that they doubted there was any need to bring in new rules.
Crews at risk
Representatives of the union, the 50,000-strong Association of Flight Attendants had told the panel that the levels of carbon monoxide, ozone and other polluting chemicals that are currently allowed put crews at risk.
Over the last nine years, the union has received reports from members reporting symptoms including headaches, nausea, and memory loss.
Judith Murawski, an industrial hygienist for the Association of Flight Attendants, which represents 27 different airlines said: "The persistence of the complaints we get from flight attendants indicate that there is a problem.
She said the majority of complaints came from staff who worked on McDonnell-Douglas MD-80 jets, which the union says has design faults which allow toxic fumes and liquids to leak into the cabin's air supply.
Levels of carbon monoxide in aircraft cabins are currently set by the US Occupational Safety and Health Association at 50 parts per million.
The union want the limits to be changed to nine parts per million, which the American Environmental Protection Agency say is safe.
'Hours' to blame
But industry scientists have said no link has ever been proven between toxic gases on planes and illnesses among the crew and that such illnesses were rare.
David Space, a Seattle-based research scientist for Boeing, said long hours and jet lag were more likely to be the causes of staff illness.
"Their duty schedule is likely to be a major factor affecting their comfort and well-being."
A national study in the US this year will look at symptoms of 6,000 air crew to see if there is a pattern.
The US Congress had also asked scientific experts to come up with new air quality regulations.
These latest fears follow concerns raised about the levels of organophosphates (OPs) from lubricants, which sometimes leak into the cabin air supply.
In September last year Costing the Earth, BBC Radio Four's environment programme, highlighted reports that some airline staff have been incapacitated in flight, and others forced to give up flying.
The UK government said it would commission a comprehensive study of the evidence.
Bruce D'Ancey, the technical secretary of the British Airline Pilots' Association, said then that he was concerned at the possible implications of lubricants using OPs, highly toxic chemicals already blamed for causing neurological damage in farmers and other people.
He says: "What we are in is a situation where we have OPs in engine oil, a known method by which they can enter the cabin through the compressor system, and we know they are hazardous to health.
"That should surely be enough to cause an investigation."
I do remember reading something about somebody saying that FA's recive 10000's of time more radiation than normal people. Hmmm...