Sulfur Dioxide (SO2) is a highly reactive gas that is produced from the combustion of fossil fuels. The largest sources of SO2 are power plants (73 percent) and other industrial facilities (20 percent). The gas is strongly linked to negative effects on the human respiratory system such as asthma. Children, the elderly, and those already with asthma are particularly vulnerable to its effects. The US Environmental Protection Agency (USEPA) says the new one-hour health standard will protect millions of citizens from short-term SO2 exposure.