The Centre has asked the states to prepare pollution control plans for nearly a dozen 'dirty' cities by August 20. Plans for a second group of cities are to be readied by October-end. As for Delhi, the petroleum ministry says the CNG queues will continue, at least till November. The Central Pollution Control Board (CPCB) estimates that between 50 and 70 per cent of the air pollution in most cities is caused by vehicles.