Aberystwyth University carried out tests over the past two years by sweeping up dust from the roadside, including the M6, near Birmingham, which carries about 80,000 vehicles a day. The sweepings contained platinum, rhodium and palladium, all of which are found in catalysts.
Scientists say the levels are still only one part per million and pose no threat, as much greater levels would be needed for any toxic effects to become apparent. But they have warned that work needs to be undertaken to make sure the metals do not become absorbed into the food chain.
Dr Nick Pearce, of the Institute of Geography and Earth Sciences at the university, who carried out the research, said: 'Catalytic converters do a good job and are effective at reducing emissions, but there is a side effect that they are creating higher levels of some metals.'