a voluntary programme has been launched by the us Environment Protection Agency ( epa ) to encourage state and local governments and businesses to retrofit older diesel engines with pollution control devices.

Under the "Diesel Retrofit Initiative,' pollution from existing heavy-duty diesel engines used in trucks, buses and construction equipment will be reduced considerably. The us epa estimates there are more than four million heavy-duty diesel vehicles operating in the us , most of them without pollution controls.

Environmentalists are satisfied with the plan. "This is a step towards tackling one of the nation's biggest and most lethal air pollution problems,' said Frank O'Donnell, executive director of the Clean Air Trust.

He, however, pointed out that the epa still needs to set high standards for new diesel trucks and buses and must eliminate as much sulphur as possible from diesel in order to prevent it from inhibiting the performance of pollution control devices.

The agency hopes to get commitments from other federal agencies, state governments, environmental groups and industries to use available technology and other methods to reduce pollution from existing diesel engines.