Pollution from Chinese coal-fired power stations is comparable to that produced by volcanic eruptions, the British Association for the Advancement of Science at Newcastle University heard last week.

Chinese coal is extremely dirty, containing many poisonous minerals. Scientists from the British Geological Survey studied an ordinary-sized plant and found that it produces 36 tonnes of arsenic, 19 tonnes of uranium and 13 tonnes of molybdenum each year.

Jane Plant, assistant director of Mineral and Geochemical Surveys at the BGS, said that much of the dangerous material is produced in the form of slag and fly ash, which, at the power station studied, is being stored. Although this keeps it away from the crops where it could enter the food chain, there is still the problem of airborne emissions. Coal burning in the home also causes pollution problems.

Pollution is even worse in northern India because the slag is used for building houses, and can easily be breathed in. "Coal-burning in China is coming in on a par with volcanic eruptions," she said.

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Professor Plant said that many diseases in the developing world could be elucidated by mapping their geochemistry. China's geochemistry, for example, means that there is a large area where the soil contains no selenium. Selenium is a trace element vital to good health: lack of it for just three months causes heart disease.

"In China there is a large area that is quite seriously affected by deficiencies in selenium," she told the meeting. "Children of ten to 12 develop heart disease problems and die. The problem is that crops grow perfectly happily and there's no way of knowing they don't have any selenium. You need geochemical maps to identify where these problems occur."

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In other countries, lack of iodine is causing 190 million cases of goitre each year and just over three million cases of cretinism. "Yet we know next to nothing about the geochemistry of iodine," she said.