City buses plying in the city of Guwahati have been found largely responsible for air pollution due to traffic. A total of 787 automobiles have been found emitting smoke out of 1330 covered by a survey carried out by the Assam Pollution Control Board in collaboration with police, during the period from June 21 to July 21. The report shows that while 81 per cent of the city buses emit smoke more than usual norm, the two wheelers and the three wheelers covered by the survey is found to have a higher smoke emission percentage in 53.02 per cent of them.