Ultra-low-sulfur diesel (ULSD) is diesel fuel with substantially lowered sulfur content. As of 2006, almost all of the petroleum-based diesel fuel available in Europe and North America is of a ULSD type. There is not a single standard set of specifications and as the government mandated standard becomes progressively more strict so does the definition. ULSD has a lower energy content due to the heavy processing required to remove large amounts of sulfur from oil, leading to lower fuel economy. Using it requires more costly oil.