Earth is being showered with mercury that can land anywhere and enter the food chain, a study has shown. The poisonous metal is released as a vapour by burning fuel then falls back to Earth and is easily absorbed by the aquatic ecosystem. Thousands of tonnes of mercury vapour are pumped into the air each year. Scientists discovered that in time mercury is oxidised it can then be deposited back on Earth, either in rain or snow. Bacteria transform the oxidised mercury into methyl mercury, which easily enters the food chain. 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 today in the journal Nature.

Dr Lyman's team of scientists analysed data from research flights over North America and Europe in October and November last year.

They used an instrument built at the University of Washington that can detect both elemental and oxidised mercury in the same air sample.

The device recorded readings every 2.5 minutes at altitudes of 19,000 to 23,000 feet. On several occasions the aircraft encountered streams of air that had descended from the stratosphere, or just below it.

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.