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Sulfur Dioxide Designations

Basic Information

The Clean Air Act requires EPA to set national ambient air quality standards for “criteria pollutants.” Currently, sulfur oxides and five other major pollutants are listed as criteria pollutants. The others are ozone, lead, carbon monoxide, nitrogen oxides, and particulate matter. The law also requires EPA to periodically review the standards and revise them if appropriate to ensure that they provide the requisite amount of health and environmental protection and to update those standards as necessary.

Sulfur dioxide (SO2) is one of a group of highly reactive gases known as “oxides of sulfur.”  The largest sources of SO2 emissions are fossil fuel combustion at power plants and other industrial facilities. Smaller sources of SO2 emissions include industrial processes such as extraction of metal from ore and the combustion of high sulfur containing fuels. 

On June 2, 2010, EPA issued a revised primary NAAQS for SO2. With this action, EPA established a new 1-hour SO2 standard at a level of 75 parts per billion (ppb), based on the 3-year average of the annual 99th percentile of 1-hour daily maximum concentrations. At the same time, the EPA also revoked both the existing 24-hour and annual primary SO2 standards.


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