Unsafe amounts of mercury have been found in four in five adults and 60 per cent of fish sold at markets in a Peruvian jungle area ravaged by illegal gold mining.
The Carnegie Institution for Science called it a "grave and mounting threat to public health" today.
Mercury is a byproduct of small-scale gold mining practiced by about 40,000 illegal miners in the Madre de Dios region on the border with Bolivia and Brazil.
The institution said women of childbearing age were worst hit - and also the most vulnerable to mercury.
The metal is a neurotoxin and can cause severe, permanent brain damage to an unborn child.
Unlike legal mining in the Andean highlands, the mining in Madre de Dios consists chiefly of scouring riverbeds and alluvial deposits for gold flecks that stick to mercury.
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