Mercury itself isn't a bogeyman, as it occurs naturally at low levels in rock, soil and water throughout the world. But about halfof all mercury released into the atmosphere today comes from the burning of fossil fuels like coal, oil and natural gas, with contributions from waste incineration, mining and other industrial activities. This mercury pollution falls directly into the ocean and other water bodies or onto land, where it can be washed into waterways. In this form, mercury poses little danger because living things can get rid of it quickly. But bacteria convert mercury as it's carried down from the ocean surface, turning it into a highly-toxic form called methylmercury.
The food chain takes it from there, as methylmercury is absorbed by phytoplankton, which are gobbled up by zooplankton, which are then feasted upon by small fish and onwards and upwards as the amount of the toxin grows in ever-accumulating quantities. The largest predatory fish in the sea, like sharks and swordfish, can have mercury concentrations in their muscles – the meat of the fish – that are 10 million times higher than those of their surrounding habitat.
Of course, the only level higher on the food chain than the largest fish is occupied by humans. And so accordingly, seafood is the source of nearly all the methylmercury that we acquire in our bodies. There's still much to be learned about the effects of methylmercury on adults – susceptibility to mercury could be genetic, or the result of diet or stress. But there is little debate that methylmercury presents the greatest risk to fetuses, infants and children, whose growing brains and nervous systems can be adversely affected.
So how do we reduce our exposure to mercury? The answer requires both difficult changes to the world's energy supply and simpler consumer choices.
When it comes to energy, the good news is that, despite major pushback from the power industry, the U.S. EPA's rule to reduce 90 percent of the mercury emissions from coal-burning power plants survived a recent Senate vote. Since half of all human-generated mercury released in the United States comes from coal power plants, that's a huge step forward towards safer fish. But the bad news is that even as the U.S. is about to choke off its mercury emissions, Asia is filling the void because two-thirds of human-generated mercury now originates from the rapidly-industrializing region. Scientists are seeing strong indications that those emissions are a major source of mercury in the North Pacific Ocean.