how would they know where it came from?

Benzene  is the main toxin in the hydrocarbon fraction of exhaust. Benzene and other less known hydrocarbons are produced in petroleum refining, and are widely used as solvents and as materials in the production of various industrial products and pesticides. Benzene also is found in gasoline and in cigarette smoke. Other environmental sources of benzene include gasoline (filling) stations, underground storage tanks that leak, wastewater from industries that use benzene, chemical spills, and groundwater next to landfills containing benzene. Exposure to benzene can cause cancer, especially leukemias and lymphomas. Benzene has a suppressive effect on bone marrow and it impairs blood cell maturation and amplification.

http://www.nutramed.com/environment/carsepa.htm