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Before 2006, most Diesel Fuel sold in the United States contained high quantities of Sulfur. Sulfur in Diesel Fuel produces air pollution emissions that are harmful to human health. In 2006, the U.S. Environmental Protection Agency issued requirements to reduce the Sulfur content of Diesel Fuel sold for use in the United States. The requirements were phased in over time, beginning with Diesel Fuel sold for vehicles used on roadways and eventually including all non-road diesel fuel. Diesel Fuel now sold in the United States for on-highway use is Ultra-Low Sulfur Diesel (ULSD), which has a Sulfur content of 15 parts per million or less. Most Diesel Fuel sold for off-highway (or non-road) use is also ULSD. To meet the 15 ppm Sulfur limit, all the streams used to prepare Diesel Fuel need hydrotreating to lower the Sulfur concentration.