The levels are low and come from polonium-210 - which is also emitted from power station chimneys, volcanoes and the ground. Dr Denis Henshaw, of the HH Wills physics laboratory at Bristol University, an expert on radon in the environment, found significantly raised levels of polonium up to 10km from motorways.
He says in the Lancet that as a result of the findings radioactivity should be added to the list of potential cancer-causing agents in vehicle exhaust.
The researchers knew radiation in children's teeth was higher in urban areas. They say the polonium-210 (210Po) in exhaust can come from leaded petrol or as a decay product of oil.
The research into levels of polonium, which emits alpha radiation, involved the analysis of 2,000 milk and permanent teeth from children. "The results are surprising in view of the number of potentially confounding factors such as the closeness of major urban areas and the fact that we do not know the length of time children have lived at their present addresses," Dr Henshaw writes.
The researchers say that excess 210Po is taken up by bone, liver and the kidneys. "At the levels detected 210Po cannot be linked with any certainty to the occurrence of cancer."
But Dr Henshaw says that there is a potential for causing cancer based on studies of high exposure to radon gas, another emitter of alpha radiation. This is associated with leukaemia, brain tumours and kidney cancer, especially in children.
A spokesman for the National Radiological Protection Board said: "There is a lot of polonium around from a variety of sources and it is difficult to see at this stage what difference this finding will make."Reuse content