Children as young as seven are working in perilous conditions in the Democratic Republic of the Congo to mine cobalt that ends up in smartphones, cars and computers sold to millions across the world, by household brands including Apple, Microsoft and Vodafone, according to a new investigation by Amnesty International.

The human rights group claims to have traced cobalt used in lithium batteries sold to 16 multinational brands to mines where young children and adults are being paid a dollar a day, working in life-threatening conditions and subjected to violence, extortion and intimidation.

More than half the world’s supply of cobalt comes from the DRC, with 20% of cobalt exported coming from artisanal mines in the southern part of the country. In 2012, Unicef estimated that there were 40,000 children working in all the mines across the south, many involved in mining cobalt.

In a joint-investigation with African Resources Watch (Afrewatch), an African NGO focusing on human rights in the minerals and extractive industries, Amnesty International says it interviewed 90 adults and children working in five artisanal cobalt mine sites. Workers spoke of labouring for 12 hours a day with no protective clothing, and with many experiencing significant health problems as a result.