The overall U.S. sulfur dioxide emissions in parts per million with a standard of 0.03 ppm. Sulfur dioxide (SO2) is a colourless gas with a sharp, irritating odour. It is produced from the burning of fossil fuels (coal and oil) and the smelting of mineral ores that contain sulfur.
Environmental Protection Agency: Sulfur Dioxide - http://www.epa.gov/oar/urbanair/so2/
Over the past 30 years, sulfur dioxide emissions have decreased by more than a third. EPA has implemented innovative, flexible programs to continue the national reductions, prevent degradation, and improve local air quality.
EPA: Sulfur Dioxide - http://www.epa.gov/oar/urbanair/so2/effrt1.html
Reducing emissions of SO2 remains a crucial component of EPA's strategy for cleaner air. Meeting EPA's national health-based air quality standards is an important step towards ensuring the air is safe to breathe. To meet the standards, EPA, states, tribes, and local governments work as partners to reduce emissions of SO2 through several control programs.