In 2006, the United States began the transition to Ultra Low Sulfur Diesel (ULSD). ULSD is defined by the United States Environmental Protection Agency (EPA) to have a maximum sulfur content of 15 parts per million (ppm). ULSD is being phased in between 2006 and 2010 and eventually will replace conventional Low Sulfur Diesel (LSD), which may contain as much as 500 ppm sulfur content. ULSD was deemed necessary to facilitate the next generation of diesel emissions control devices.
The transition to ULSD has been relatively smooth with more than 95% of the on-road diesel fuel meeting the ULSD standard with an average sulfur content of 7 - 8 ppm sulfur. One of the biggest challenges remaining is the enforcement of the ULSD labeling requirements for retail fuel dispensers.
In 2010, all on-highway diesel fuel will be required to be ULSD compliant.