But the risk of mercury exposure in gold mining "cannot be solved through a ban," said Perrez, who called that aspect of the negotiations "a special situation" that requires a more complex approach.

Mercury concentrations pose the greatest risk of nerve damage to pregnant women, women of childbearing age and young children.

As a naturally occurring element, mercury comes from the earth's crust and, like some other elements, cannot be created or destroyed. Some natural processes, like volcano eruptions and weathering of rocks, release mercury into the environment. But about 30 percent of mercury emissions come from human causes, which the treaty would seek to reduce.

Once it gets into the land, air and water, mercury accumulates in fish and wildlife and goes up the food chain. Most of it isn't removed until ocean or lake sediments bury it, or other mineral compounds trap it.

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