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In the 1920s lead was added to petrol. It made cars more powerful and was, according to its advocates, a "gift". But lead is a gift which poisons people; something figured out as long ago as Roman times. There's some evidence that as countries get richer, they tend initially to get dirtier and later clean up. Economists call this the "environmental Kuznets curve". It took the United States until the 1970s to tax lead in petrol, then finally ban it, as the country moved down the far side of the environmental Kuznets curve. But as Tim Harford explains in this astonishing story, the consequences of the Kuznets curve aren't always only economic.

Presenter: Tim Harford
Producer: Ben Crighton.