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“I GET excited every time I see a street cleaner,” says Hazel Prichard. It’s what they collect in their sacks that gets her juices flowing, because the grime and litter they sweep up off the streets is laced with traces of platinum, one of the world’s rarest and most expensive metals. The catalytic converters that keep exhaust pollutants from cars, trucks and buses down to an acceptable level all use platinum, and over the years it is slowly but steadily lost through these vehicles’ exhaust pipes. Prichard, a geologist at the University of Cardiff in the UK, reckons that tonnes of the stuff is being sprayed out onto the world’s streets and highways every year, and she is hunting for places where it is concentrated enough to be worth recovering. One of her prime targets is the waste containers in road-sweeping machines.

This could prove lucrative, but Prichard is motivated by something far more significant than the chance of a quick buck. Platinum is a vital component not only of catalytic converters but also of fuel cells – and supplies are running out. It has been estimated that if all the 500 million vehicles in use today were re-equipped with fuel cells, operating losses would mean that all the world’s sources of platinum would be exhausted within 15 years. Unlike with oil or diamonds, there is no synthetic alternative: platinum is a chemical element, and once we have used it all there is no way on earth of getting any more. What price then pollution-free cities?

It’s not just the world’s platinum that is being used up at an alarming rate. The same goes for many other…