An official website of the United States government.

This is not the current EPA website. To navigate to the current EPA website, please go to This website is historical material reflecting the EPA website as it existed on January 19, 2021. This website is no longer updated and links to external websites and some internal pages may not work. More information »

Sulfur Dioxide Designations

EPA Completes Third Round of Sulfur Dioxide Designations

On December 21, 2017, the U.S. Environmental Protection Agency (EPA) took an additional step to implement the health-based national air quality standard for sulfur dioxide (SO2) set in 2010. EPA has identified or “designated” areas:
  • “nonattainment” – not meeting the 1-hour health-based national air quality standard for SO2 (six areas in three states and two territories),
  • “unclassifiable” – not enough information to make a determination (23 areas in ten states), or
  • “attainment/unclassifiable” - meeting the standard and not contributing to a violation of the standard (all remaining areas except where additional SO2 air quality monitoring is underway).

Read the Federal Register Notice:  Air Quality Designations for the 2010 Sulfur Dioxide (SO2) Primary National Ambient Air Quality Standard - Round 3

Read the Technical Support Documents supporting this action.

Note: On March 28, 2018, the EPA withdrew the designation of Unclassifiable for the Citrus County, Florida, area that the EPA originally issued on December 21, 2017, and established a designation of Attainment/Unclassifiable for that area. Learn More

You may need a PDF reader to view some of the files on this page. See EPA’s About PDF page to learn more.