Coal ash contains low concentrations of arsenic, mercury and other heavy metals, although it's not classified as hazardous.

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It said its safety tests showed that plane engines had "increased tolerance levels in low ash density areas".

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The decision to lift the restrictions followed safety tests that showed the engines could cope in areas of low-density ash.

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Further, the coal is low on sulfur and fly ash content, which increases its attractiveness.

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So though the total volume of the eruption, put at about 0.14 cubic kilometres, is low, the amount of ash capable of travelling long distances is high.

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Passenger numbers at Cardiff Airport dropped by 200, 000 last year, with the airport blaming a cut in capacity by a low-cost airline and the ash cloud.

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