Natural and artificial platinum and palladium occurrences worldwide.

The introduction of legislation in the western world over the last 20 years or so requires that poisonous exhaust emissions from cars are reduced. Catalytic converters fitted to exhaust systems make these poisonous gases safer. Now nearly half the world's annual production of platinum (Pt) and palladium (Pd) is being used in the manufacture of catalytic converters. As cars travel around our cities these converters lose Pt and Pd which are ejected onto roads. This creates artificial concentrations of Pt and Pd in the urban environment that is a new addition to the global distribution. We know very little about such concentrations but we know much more about Pt and Pd concentrations in geological settings. Pt and Pd occur naturally at the Earth's surface only in a very few rare locations in rocks formed by an unusual combination of geological processes. The major Pt and Pd deposits were formed by crystallisation of magma which concentrated the metals into specific minor rock units within large igneous intrusions. As these precious metals are very rare on the Earth's surface they are economic at concentrations of only a few parts per million. Recent studies, including those described here, show that values of Pt and Pd accumulating in our cities are approaching values found in natural deposits. Certainly Pt and Pd can be located in road dust at road junctions in the cities in the wealthy western world at levels well above natural background values