The new mercury treaty will rewrite the rules on how the toxic element can be used around the world. But it remains unclear how the treaty will affect those facing some of the greatest health risks from the substance: small-scale gold miners in the developing world.

“The scientific evidence is so incontestable … and the health impacts are so debilitating,” says Achim Steiner, executive director of the UN Environment Programme (Unep). “We want to find a way in which mercury can be taken out of the small-scale gold mining sector. Unbeknown to many of the people who are engaged in this gold mining, it is a very harmful compound.”

The mercury treaty, which was finalised in Geneva after a five-day negotiating marathon that finished early on Saturday morning, will regulate the supply, trade and use of the substance. Under the terms of the treaty, governments will be encouraged – but not obliged – to take measures to manage the health impacts of mercury exposure. In countries where artisanal gold mining takes place, governments will be required to draft national action plans to reduce its use.

Mercury has long been used in thermometers, dental fillings, batteries and fluorescent lamps. The metallic element can also be a by-product of industrial processes including coal combustion, cement production and waste incineration. But, according to Unep (pdf), the biggest source of global mercury pollution is small-scale gold mining, an activity that sustains the livelihoods of as many as 15 million people in 70 countries, mostly poor ones.

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