The Environmental Protection Agency (EPA) has issued further guidelines on its drive to tighten clean air controls.

One of the primary elements of the announcement was the EPA's 'Clean Air Non-road Diesel Rule', which will regulate emissions from construction and other non-road equipment powered by diesel engines.

The rule also cuts sulphur levels in diesel fuel by more than 99 per cent over current levels, which the EPA says will "significantly help localities achieve cleaner air".

Meanwhile, the agency also sought to criticise more than 450 counties in the US which have so far yet to meet standards governing ozone emissions.

EPA Administrator Mike Leavitt said the continuing drive to tighten emissions regulations, for both the automotive industry and non-vehicle sources, was part of a campaign aimed at improving the health of both the nation and the environment.

"The air is getting cleaner. These new rules are about our new understanding of health threats; about our standards getting tougher and our national resolve to meet them," he declared.

The suite of inter-related actions all fall under the Clean Air Rules of 2004, which detail national tools to help states and communities meet the national standards.

Increasing severity of emissions legislation implies a rise in demand for platinum group metal autocatalysts to control harmful emissions, particularly in the diesel automotive sector, in which platinum catalysts are used to control harmful exhaust emissions.