The leading US oil refiners association has announced that it will sue the Environmental Protection Agency over new rules concerning the level of sulphur in diesel.The National Petrochemical & Refiners Association (NPRA) said on 23 January that it will challenge the outgoing EPA's final diesel sulphur rule in federal court which proposes to cut sulphur in diesel fuel to 15 parts per million (ppm) compared to current levels of 500 ppm.