The Taj Trapezium Zone (TTZ) Authority has banned petrol and diesel vehicles within 500 metre of Taj Mahal, the government said in the Lok Sabha today. In a written reply, Union Environment Minister Harsh Vardhan said a study had found that the white marble of the historic monument was turning yellow due to carbon pollution. Thus, the TTZ Authority had ordered the light petrol and diesel vehicles operating around the Taj Mahal to switch to CNG, he added. "The Taj Trapezium Zone Authority has banned petrol and diesel vehicles within 500 metre of the Taj Mahal. It has passed orders for conversion of the light petrol and diesel vehicles operating around the Taj into CNG," said the minister. Every effort was being made by the authorities concerned to ensure compliance of the orders and a number of vehicles had been issued challans for violations, he added. Harsh Vardhan said numerous studies had been carried out by various research teams on the discolouring of the Taj Mahal. According to one such study, carbon pollution around the monument was causing its white marble to turn yellow, he added.