The sustainability of cobalt is an important emerging issue because this critical base metal is an essential component of lithium-ion batteries for electric vehicles. More than half the world's cobalt mine production comes from the Katanga Copperbelt in DR Congo, with a substantial proportion (estimated at 15-20%) being extracted by artisanal miners. Here we show, in a case study performed in the town of Kolwezi, that people living in a neighbourhood that had been transformed into an artisanal cobalt mine, had much higher levels of cobalt in urine and blood than people living in a nearby control area. The differences were most pronounced for children, in whom we also found evidence of exposure-related oxidative DNA damage. It was already known that industrial mining and processing of metals have led to severe environmental pollution in the region. This field study provides novel and robust empirical evidence that the artisanal extraction of cobalt that prevails in the DR Congo may cause toxic harm to vulnerable communities. This strengthens the conclusion that the currently existing cobalt supply chain is not sustainable.