US authorities have marginally relaxed clean air rules for diesel emissions, allowing higher levels of sulphur.

Diesel fuel produced in the US will now be permitted to have up to 18 parts per million (ppm) of sulphur in it, up from the 17ppm currently allowed under rules laid out by the US Environmental Protection Agency (EPA).

The EPA said that the change was only temporary, adding that it was necessary until more precise tests were in use throughout the industry measuring sulphur emissions.

Sulphur emissions were due to be capped to 15ppm at the start of June with a tolerance level of up to 17ppm. The widening of this to 18ppm will remain in place until October 14th 2008.

"This technical change will help ensure that fuel actually meeting the 15ppm sulphur cap is not falsely rejected," it said in a statement.

It added: "The adjustment provides labs with more time to improve their measurement techniques."

The EPA said that its decision would have no environmental impact.

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