Petro/diesel used in automobiles contains tetraethyl lead, (C2H5)4Pb. Tetraethyl lead acts as an anti-knocking agent. Anti-knocking agent such as, tetraethyl lead is added to petrol/diesel to make its combustion smoother. During combustion, tetraethyl lead decomposes to produce lead compounds, which are poisonous.
Thus, poisonous lead compound are present in the exhaust of automobiles. These lead compounds pollute the air, get absorbed in plant and then enter to our bodies through food chains.