Exelon, one of the largest US electricity groups, has backed tighter federal standards for pollution control, saying they can be met without excessive damage to energy supplies by the increased use of natural gas for power generation. That sets Exelon against other leading US power groups, which have warned that new regulations will drive up electricity prices and could threaten the reliability of supplies...Two proposed EPA emissions regulations have been particularly controversial: the cross-state air pollution rule, which limits sulphur dioxide and nitrogen oxides that cause acid rain; and the air toxics rule, which controls pollutants such as mercury and arsenic. [also refers to Southern Company, Luminant (part of Energy Future Holdings)]