Air pollution laws in the United States have reduced the acidity of rainfall in parts of the northeast, mid-Atlantic, and midwest regions, but there is still a long way to go in reducing emissions from older coal-burning power and industrial plants. Some plants have lowered SO2 emissions by switching from high-sulfur to lowsulfur coals. However, this has increased CO2 emissions that contribute to global warming, because low-sulfur coal has a lower heat value, which means that more coal must be burned to generate a given amount of electricity. Low-sulfur coal also has higher levels of toxic mercury and other trace metals, so burning it emits more of these hazardous chemicals into the atmosphere.