According to the latest figures from the United Nations Environment Programme (UNEP), artisanal small-scale gold mining is now the largest single contributor of mercury pollution in the world, accounting for around 37% of anthropogenic emissions in 2010. Next are fossil fuel combustion, predominantly by coal-fired power plants, metal and cement production, waste incineration and others.
After it's emitted into the atmosphere, where it spreads around the globe, mercury (which is also released by natural sources such as volcanoes, hot springs and erosion) is oxidised, deposited on the earth’s surface and converted into toxic methyl mercury (MeHg) by bacteria. MeHg is bio-available, meaning that it’s absorbed into the bodies of animals that ingest it, and accumulates especially in the marine food chain with apex predators like tuna, swordfish, sharks, seals, beluga whales and polar bears ending up most contaminated.
http://www.news24.com/Columnists/AndreasSpath/Heavy-metal-is-bad-for-you-20130204
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