The use of unleaded petrol may have caused the atmospheric lead levels in Delhi to drop by as much as 61 per cent, but this petrol brings with it an added threat: cancer. The unleaded petrol being used in the Capital is high on the carcinogen benzene, which is added to prevent the engine from knocking."Continued exposure to the particulates in benzene can cause disability, pulmonary fibrosis, cancers of the blood, skin and lungs and circulatory disorders," warns Dr Naresh Trehan, Director, Escorts Heart Institute.