The city of London now heads the list of the world's cleanest big cities, according to a British government report. The city has moved ahead of Tokyo, New York and Los Angeles on the list. The study, conducted by researchers at the Imperial College, London, states that the risks from air pollution caused by traffic are 1,000 times lower than those associated with smoking and the level of pollution due to vehicular emissions is falling throughout the uk . Green groups said the government was using the study in order to renege on its commitment to reduce traffic congestion. Green group Friends of the Earth also published a report arguing that air quality in the uk last year showed the biggest drop since the city began to maintain records in 1993.