American Airlines Flight 109, traveling from London to Los Angeles, was two hours into its flight when passengers and crew members suddenly started fainting or otherwise becoming ill.   The captain turned the Boeing 777 around and landed at Heathrow. American Airlines 777 

According to the Daily Telegraph in Britain, the aircraft likely experienced a problem with the

The air in an airliner’s cabin has been compressed by the aircraft’s engines. Most of the time, the air is safe to breathe. But if a leaky seal in the engine allows the air to first mix with heated engine oil, the cabin air can be contaminated with toxic fumes.

When a cabin fills with toxic chemicals from an

A cabin suddenly fills with fumes.  Passengers get ill.  The fumes eventually clear.  But for some, the symptoms persist long after the flight is over.  Others will first develop symptoms weeks or months later, and may not even relate their symptoms to their flight.  These passengers are all the victims of what has become known as a "fume