Sulfur dioxide (SO2) is one of a collection of highly reactive gases known as oxides of sulfur (SOX). Over 90 percent of SO2 emissions come from fossil fuel combustion at power plants and other industrial facilities. Smaller sources of SO2 include industrial processes like extracting metal from ore and, to a lesser extent, the burning of high-sulfur fuel by locomotives, marine engines and nonroad engines and equipment, as well as onroad mobile sources, fires, waste disposal, residential wood combustion and solvent use.
Exposure to SO2 is associated with a range of adverse health effects, including those related to the respiratory system, such as a narrowing of the airway causing breathing difficulty, particularly during exercise; aggravated asthma symptoms; and increased emergency room visits and hospital admissions for respiratory illnesses and asthma. Children, older adults and those with asthma are most at risk of suffering health impacts from exposure to SO2. SO2 can also contribute to the acidification and eutrophication of water bodies.
SO2 is often emitted along with other SOX, which react with other chemicals in the air to form fine particulate matter (PM2.5), which is also an extremely harmful pollutant (see related section on particle pollution).
EPA’s last review of the National Ambient Air Quality Standards for SO2 culminated in June 2010 with the agency replacing the existing annual and 24-hour primary (to protect public health) SO2 standards with a new 1-hour standard of 75 parts per billion (ppb), consistent with new scientific findings providing evidence of a link between short-term SO2 exposures, ranging from 5 minutes to 24 hours, and adverse respiratory impacts. The agency is now engaged in the review of these standards and expects to issue a decision on whether or not to revise them in late 2017.