New analysis of data from the US Energy Information Administration has shown that between 1997 and 2017, as coal firing in the US fell from 2006 million MWh of electrical power generation per year to 1206 million MWh (a 40% decrease), SO₂ and NO_x emissions from US power generation fell by 88% and 76% respectively. Over those 20 years, coal firing produced 90% of SO₂ emissions and 76% of NO_x. As these figures suggest, the rate of emissions of SO₂ and NO_x­ from coal-firing has decreased markedly during the period – SO₂ generation is now almost 84% lower, while NO_x is almost 77% lower than in 1997. As the Power Engineering article explains, the causes of the reductions were twofold – higher efficiency plants coupled with better pollutant reduction technologies. The driver for the second cause was regulatory, with Clean Air Act amendments such as the Acid Rain Program and the Mercury and Air Toxic Standards being major contributors, supplemented by the Clean Air Interstate Rule, which became the Cross-State Air Pollution Rule in 2015, which affected 27 states.