Mercury

The Earth's atmosphere contains about 2500 tonnes of mercury and this has increased down the centuries. Some of this is natural, coming from volcanoes, soil erosion and microbial release of methylmercury, but most comes from human activity, mainly from burning coal.

Humans have been using mercury for thousands of years, having discovered that when the red pigment cinnabar (HgS) was heated strongly, it left behind a shiny liquid metal, quicksilver. This had the ability to dissolve gold and so it could be used to extract it from alluvial deposits. Mercury continued to be used in this manner for centuries, including in Brazil as late as the 1990s. Today we know that this metal is inherently dangerous.