Earth is being showered with mercury from the upper atmosphere that can land anywhere and enter the food chain, a study has shown.

The poisonous metal is released as a vapour by burning fuel before falling back to Earth in a form that is easily taken up by aquatic ecosystem - the ecosystem in a body of water.

Thousands of tonnes of mercury vapour are pumped into the atmosphere each year.

Scientists discovered that over time the elemental mercury is oxidised by chemistry in the upper troposphere and lower stratosphere. It can then be deposited back on Earth, either in rain or snow or after being carried on air currents.

Bacteria transform the oxidised mercury into methyl mercury, which can easily enter the food chain and contaminate fish.

US scientist Dr Seth Lyman, who led the research while at the University of Washington Bothell, said: "Much of the emitted mercury is deposited far from its original sources. Mercury emitted on the other side of the globe could be deposited right at our back door, depending on where and how it is transported, chemically transformed and deposited."

Mercury from coal burning in Asia, for example, could circle the globe several times before being oxidised and carried back to the Earth's surface.

Some areas, including the south-west US, had specific climate conditions that allowed them to receive more oxidised mercury from the upper atmosphere than others.

The findings are reported in an early online edition of the journal Nature.

The results showed that elemental mercury is turned into oxidised mercury in the upper atmosphere. How the oxidation process takes place is not clear. But scientist know that once it occurs the mercury can return to Earth through precipitation or air moving to the surface. "The upper atmosphere is acting as a chemical reactor to make the mercury more able to be deposited in ecosystems," said Dr Lyman.