Sulfur Dioxide (SO2)
Note: EPA no longer updates this information, but it may be useful as a reference or resource.
Please see www.epa.gov/airtrends for the latest information on Air Quality Trends.
Nature and Sources of the Pollutant: Sulfur dioxide belongs to the family of sulfur oxide gases. These gases are formed when fuel containing sulfur (mainly coal and oil) is burned, and during metal smelting and other industrial processes. Most SO2 monitoring stations are located in urban areas. The highest monitored concentrations of SO2 are recorded in the vicinity of large industrial facilities.
Health and Environmental Effects: The major health concerns associated with exposure to high concentrations of SO2 include effects on breathing, respiratory illness, alterations in the lungs' defenses, and aggravation of existing cardiovascular disease. Major subgroups of the population that are most sensitive to SO2 include asthmatics and individuals with cardiovascular disease or chronic lung disease, as well as children and the elderly.
Together, SO2 and NOx are the major precursors to acidic deposition (acid rain), which is associated with the acidification of lakes and streams, accelerated corrosion of buildings and monuments, and reduced visibility. Sulfur dioxide is a major precursor to PM2.5, which is of significant concern to health as well as a main pollutant that impairs visibility.
Trends in SO2 Levels: Between 1987 and 1996, national SO2 concentrations decreased 37 percent and SO2 emissions decreased 14 percent. Between 1996 and 1996, national SO2 concentrations remained unchanged and SO2 emissions increased 3 percent. Sulfur dioxide emissions from electric utilities decreased 20 percent between 1987 and 1996. Since 1993, SO2 emissions from electric utilities decreased 17 percent. These recent reductions are due, in large part, to controls implemented under EPA's Acid Rain Program. The 4 percent increase that occurred between 1996 and 1996 is primarily due to increased demand for electricity.