E-bike enthusiasts brag they can avoid breaking a sweat and pumping out emissions all at once. But they cannot claim that mass adoption of the bikes in China has left no trace. A recent study by Wood Mackenzie, an energy research firm, estimates that roughly 15 per cent of global lead production is used in making e-bike batteries for Chinese consumers.

In five years of explosive growth up to 2013, lead demand grew by half to 4.8m tonnes. About three-fifths of that was due to e-bikes. There were more than 200m e-bikes on China’s roads last year. Their popularity reflects their convenience as a cheap mode of transport for residents of fast-developing cities.

The lead in the batteries is easily recycled. But to boost affordability, batteries are often made cheaply without much regard for the environment. A recent government push toward a clean-up caused dirty lead smelters and battery makers to shut down.