questionText: To ask the Secretary of State for Transport, if he will make a comparative estimate of the average contribution to air pollution of (a) vehicles on the M4 and (b) air traffic at Heathrow.
answerText: The Department for Transport has no plans to make a comparative estimate of the average contribution to air pollution from vehicles on the M4 and from air traffic at Heathrow. 
heading: Heathrow Airport and M4: Air Pollution