A doctor who tended the sick on a flight which was forced to turn back when cabin crew were taken ill says the sick reported smelling burning on-board.

Dr Barbara Persons was a passenger on flight AA109 from London Heathrow to Los Angeles and offered to help after cabin crew and fellow-flyers reported feeling dizzy.

The aircraft was forced to turn around over the Atlantic after the captain called a 'medical emergency' as increasing numbers of people on-board became ill.

Dr Persons, an American surgeon who had been in Europe for a medical conference, told MailOnline: 'The flight attendant had passed out just outside of the business class lavatory and I attended to her first.

'She awoke after about a minute. Then lost consciousness again before being placed on oxygen. Five other flight attendants and one passenger, an older gentleman who had recently had an aortic valve replacement, all felt ill.'

Scroll down for video

Aparamedic from London Ambulance Service, Hazardous Area Response Team, carrying out tests on the air inside the cabin of a flight forced to turn back to London over the Atlantic after crew starting fainting

Emergency services were called to the plane after it landed back in London. Tests were then carried out on the air quality on board

She added: 'I placed them on oxygen, assessed all and talked with captain Frank McGill. He made the immediate decision to fly back to Heathrow.

'The second flight attendant was dizzy and flushed and unable to stand. Other flight attendants all toward front of plane were dizzy, light headed, had vertigo and were slightly nauseated. Two said they smelled a slight electrical or burning smell.'

Dr Persons, from Lafayette, California, praised the captain's actions and the attitude of the ill crew, who she said apologised for not being at their posts.

She said: 'He made the right decision. If it had have been something coming from a bag or carbon monoxide, and we had carried on, we all could have died.'

Dr Barbara Persons, an expert in plastic and reconstructive surgery, stepped forward to help the growing number of sick during the flight

Investigators are this morning trying to find out what caused the sudden bouts of illness.

Emergency vehicles were scrambled when the plane landed back in Heathrow and the aborted flight was escorted to a terminal, where passengers' bags were checked.

Those on-board were not allowed to disembark, nor paramedics allowed on to treat the sick, until tests had been carried out for any 'elevated levels of substances' in the cabin air.

Experts have said there are a number of possible causes for the incident, including a problem in the galley where air crew cook food, a fault with the air conditioning system or a chemical leak from a passenger's bag.

An aviation specialist said there is a chance that an oven or another piece of equipment was giving off fumes in the galley, either due to a fault with the equipment or due to the chemicals used to clean it.

Another possible cause is that a laptop battery or another electrical device leaked in a passenger's bag, producing fumes.

Those taken ill were said to have felt faint some 1,600miles into the 5,500mile flight last night.

A spokesman for American Airlines said this afternoon: 'Our maintenance team conducted a thorough inspection of the aircraft, including a test flight, and found no issues with the Boeing 777-300. Out of an abundance of caution, all of the air filters on the aircraft were replaced.'

Passengers have told of the dramatic moment people mysteriously fell ill onboard a Los Angeles-bound flight from Heathrow. The aircraft declared an emergency and was met by emergency vehicles in London (pictured)

A passenger onboard, Eric Winter, tweeted these pictures of emergency services surrounding the aircraft

Aviation expert and pilot Bruce Rodger told MailOnline that the captain did the right thing by turning the flight around.

'Anytime there is a problem in flight when passengers are ill - especially five or six - the captain will most likely turn the plane around to land it at the closest place,' he said.

Mr Rodger, who has over 20 years of flying experience especially with several different Boeing 700-model type of planes, explained that since the flight was in it's early stages of journey that food poisoning is not likely a factor as to what caused people to get sick.

Instead, he said that in this case, air quality could be a question.

'Air quality on the air plane is one thing I would consider. This situation could also just be circumstance,' Rodger said.

'It sounds like it was bad and this could have been a situation that the air quality was possibly compromised somehow.

'The air on that type of plane is completely clean for passengers and they are always getting clean quality air.'

Aviation expert and pilot Bruce Rodger (above) told MailOnline that the captain did the right thing by turning the flight around.

The airline said last night that passengers were being reunited with their bags, but a spokesman would not reveal what checks had been carried out or whether anything had been found.

He added: 'American Airlines, and Heathrow authorities, were inspecting bags and cargo', before revealing that the airline's maintenance team was now carrying out a 'thorough inspection of the aircraft.'

WHAT COULD HAVE CAUSED THE SUDDEN REPORTS OF SICKNESS? 1. Problem in the galley. It is possible that the flight crew felt sick due to a problem with a machine in the galley. An oven may have been smoldering or could have been cleaned with a chemical which, when the oven was switched on, gave off fumes. This may explain why the flight crew reported feeling sick while only two passengers were taken ill. 2. Fault with the air conditioning pack. All planes use an air-conditioning system which maintains the pressure and quality of the air. Research has stated that the air quality on most airliners is better than in a car, but faults have been known with the packs. However, experts say that such faults are usually immediately obvious as the system would pump out blue smoke, and it would affect everyone in the cabin, rather than just a few people. American Airlines has said it found no fault with the aircraft. 3. Something in a passenger's bag. Passengers on the flight reported having their bags checked after landing. It is possible a faulty laptop battery may have leaked a chemical or that someone had another substance which leaked, caused people to feel nauseous. 4. Hysteria. Experts say that people get a heightened sense of concern about their health once they see others feeling sick. It is possible the fainting of the air hostess caused others to become concerned about their health and report feeling ill with no direct cause. Advertisement

Alan Gray, 41, who told of how flight AA109 was given an escort by emergency services as it taxied to the terminal, said he and his fellow passengers had not been given an explanation about why their bags were confiscated.

'They wouldn't let us have our luggage. They're doing checks on it so it looks like there could be something more to it,' the band manager told MailOnline.

'When we landed there were police cars, ambulances and firefighters who escorted us down the runway and then we were held for 45 minutes before docking.

'Eventually, when we did dock, there was only one person who came on and he was monitoring the air.

'Then the paramedics were allowed on-board to treat those who were ill and everybody was let off.'

Mr Gray was onboard the flight with boyband Race the Horizon, who were finalists on Britain's Got Talent in 2012.

One of the members, Kris Evans, 25, told MailOnline: 'It was just a bit mad. We didn't really know what was going on - initially we thought it was to do with the air conditioning.

'A cabin crew lady fell within touching distance of where we were sitting and then another guy next to me looked a bit iffy.

'There were around three passengers who fell ill.'

Mr Gray added: 'One of the flight attendants was walking down the aisle when she collapsed. Then up to six other cabin crew members said they felt light-headed and as though they were going to faint.

'It was at that point the captain said he was turning the plane around. He said he wasn't willing to take the risk to keep going and hadn't got the crew to do it.

'Then three passengers became really unwell too, and a few others were turning a bit pale. It was a bit strange we didn't just land in Iceland.'

The band Race the Horizon were on-board the flight. Their manager Alan Gray (left) told of how the plane was given an escort by emergency services. The band are, from right, Jay Kontzle, Kris Evans and Craig Worsley

Response: Passenger Eric Winter, who was in London on business with UFC Fight Pass, tweeted this picture of the aircraft surrounded by emergency vehicles after landing back at Heathrow on Wednesday evening

Mystery: Two passengers are believed to have felt faint, including a man said to be in his 60s and another in his 40s. Six crew members were also said to have felt unwell (pictured is the aircraft landing)

Meanwhile, another passenger Lee Gunn told Mirror Online: 'About 2.5 hours into the flight just as we were passing Iceland we had a Tannoy announcement asking for any doctors, nurses or medical professionals on board to report to the boarding doors to assist with unwell passengers.

'The lights then came on in the cabin and there was lots of commotion.

'It was also reported that 7 of the crew - 13 on board in total I believe - had fallen ill, along with "many' passengers".'

WERE YOU ON-BOARD THE FLIGHT? Were you on-board the aborted flight? Call 0203 615 2676 or email richard.spillett@mailonline.co.uk Advertisement

The AA109 flight left Heathrow at midday on Wednesday and was due to arrive in LA later that day.

The Boeing 777-300 was close to Keflavik, Iceland, when it returned to its origin, some four hours into the flight. It landed in London shortly after 5pm.

A specialist team from London Fire Service was then on hand to carry out tests for 'elevated levels of any substances', a spokesperson said.

She added: 'Everything was checked and tested but there were no readings for any substances of any sort that may have been a problem.'

The spokesperson said firefighters had 'no idea' what may have caused people to fall ill.

The London Ambulance Service tweeted: 'We have attended an incident at #HeathrowAirport today. We checked over six patients who were feeling unwell. They were discharged on scene.'

Were you on the flight? Call 0203 615 2676 or email richard.spillett@mailonline.co.uk

Emergency: Those on board the American Airlines flight AA109 have described how they were then kept in the dark about what was happening as the aircraft declared an emergency and returned to London (pictured)