Over the next 25 years the regulations - which cover 40,000 marine vessels and nearly 21,000 diesel locomotives - will cut these engines' annual emissions of nitrogen oxide, a key ingredient in smog, by 80% and fine particulate matter, or soot, by 90%.

EPA officials estimated that by 2030 the health benefits associated with the new standards will outweigh the costs by 20 to 1, preventing 1,500 premature deaths and 1,100 hospitalisations a year.

‘By tackling the greatest remaining source of diesel emissions, we're keeping our nation's clean-air progress moving full steam ahead,’ said EPA administrator Stephen L Johnson. ‘Over the last century, diesels have been America's economic workhorse, and through this rule, an economic workhorse is also becoming an environmental workhorse.’

The EPA rule is titled Control of Emissions from Non-Road Spark Ignition Engines and Equipment. The proposal requires spark-ignited marine engine manufacturers to meet new emission standards beginning in 2009 and requires boatbuilders to reduce evaporative emissions from boat fuel systems.

Under the proposal, outboard and PWC engines will have to be certified to the same stringent exhaust emission standards as will be required by the California Air Resources Board in 2008.

For sterndrive and inboard engines, the EPA rule proposes catalyst-based exhaust emission standards apply beginning in 2009. Boatbuilders will be required to change their fuel systems with requirements for fuel hose, plastic fuel tanks and controlling emissions from the fuel tank vent.

‘The US industry has worked closely with the EPA on this,’ Tony Rice, secretary general of the International Council of Marine Industry Associations (ICOMIA) told BB. ‘As a consequence recreational craft are not required to have any form of after treatment and thus these new limits will be achieved through natural evolution of in-cylinder design.’

However, he added, we are working with the NMMA to ensure that we have a clear understanding of the definition of recreational craft to ensure that charter yachts are not classified as commercial and thus fall under the after-treatment requirements.