The nation's refineries, as of June 1, have begun producing low-sulfur diesel fuel for use in off-road equipment, including boats.

The new fuel will then wind its way through the distribution system, so that by December 1, locomotives and marine vessels, as well as farming, construction and mining equipment will switch to diesel containing nearly 85 percent less sulfur.

This new off-road fuel has a sulfur content of 500 parts per million, reduced from about 3,000 parts per million in the previous blend.

"June 1 marks an important next step on the road to clean diesel. Last year saw the nationwide availability of on-highway ultra-low-sulfur diesel fuel enabling manufacturers to engineer the cleanest diesel trucks ever. This new fuel will begin bringing the same benefits to off-road equipment," Allen Schaeffer, executive director of the Diesel Technology Forum, said in a statement.

By 2010, sulfur levels in most non-road diesel fuel will be reduced to 15 parts per million, making it possible for engine manufacturers to use advanced exhaust control systems that significantly reduce emissions.

For locomotive and marine fuel, this step will occur in 2012, according to the Diesel Technology Forum.