Its reported a flight attendant fainted and that five other crew members fell ill.
Three passengers aboard the flight also reported to be feeling faint.
The plane declared an emergency around 4pm (London time) where it then returned to Heathrow.
When arriving at LHR, passengers luggage was confiscated for checks.
Passenger Alan Gray told the Daily Mail, of how flight AA109 was escorted by emergency services to the terminal and said himself and his fellow passengers had not been given an explanation.
He also said they “they would let us have our luggage. They’re doing checks on it so it looks like there could be something more to it”.
He said that once the flight had landed, there was police cars, ambulances and firefighters who escorted the flight and was held for 45 minutes.
Kris Evans told the Daily Mail that he didn’t know what was going on and thought it was related to the air conditioning.
He said a flight attendant had fell within touching distance of where he was sitting and then another guy next to him looked ‘iffy’.
One flight attendant collapsed while walking down the aisle.
The flight safely landed at Heathrow at 5pm.