Heathrow Airport chief executive Colin Matthews says: "We say a hub (airport) matters but actually does it matter to the country as a whole?
"The economic analysis we published today suggests it really does."
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Official readings are recorded three metres in the air but a study found levels of nitrogen dioxide (NO2) were higher at pushchair level.
Tubes, trains, trams, buses, cable cars, river buses, taxis and going by foot - your guide to getting around during the strike.
Union officials rejected a revised offer from Underground managers during a meeting at the conciliation service ACAS this afternoon.