The majority of the pollution is caused by vehicles on the road and experts say reducing traffic in the cities is the best way of improving air quality.

BBC: Air pollution fears in Leicester and Nottingham

That warning is followed by sounds of air traffic on the cockpit recording.

NPR: Moussaoui Jury Hears Tapes of Flight 93 Crash

On the tape, Sullenberger is heard issuing a "mayday" call and telling air traffic controllers the flight had lost power in both engines, Higgins said.

CNN: Investigators find feather, other evidence on downed plane

The real problem is an antiquated air-traffic control system "that is essentially based on World War II-era technology, " says Victoria Day, a spokeswoman with the Air Transport Association, the industry's lobby group.

NPR: Attention, Airlines: This Is Your Passenger Speaking

He said he was determined to "face up to the necessity of reform", an example of which is the part-privatisation of the National Air Traffic Control System (Nats) to be debated by the Commons on Tuesday.

BBC: You are in: UK Politics