SO₂ (2010) NAAQS Notes

More detailed design value data can be found on the Air Quality Design Value site.

1. The 2010 NAAQS for SO₂ is a 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. The previous 24-hour and annual primary SO₂ standards remain in effect until the effective date of an area's designation for the 2010 1-hour standard.

2. A group of 29 areas were designated nonattainment for the 2010 SO₂ NAAQS effective October 4, 2013.
   
   Effective September 12, 2016, four additional areas were designated nonattainment: Alton Township IL, Williamson County IL, Anne Arundel County and Baltimore County MD, and St. Clair MI. See: 81 FR 45039
   
   Effective January 17, 2017, three additional areas were designated nonattainment: Freestone and Anderson Counties TX, Rusk and Panola Counties TX, and Titus County TX. See: 81 FR 89870
   
   Effective April 9, 2018, six additional areas were designated nonattainment: Hillsborough-Polk County FL, Huntington IN, Evangeline Parish (Partial) LA, Piti-Cabras GU, San Juan PR, Guayama-Salinas PR. See: 83 FR 1098
   
   Effective April 30, 2021, nine additional areas were designated nonattainment: Henderson-Webster Counties, KY, New Madrid County, MO, St. Lawrence County, NY, Howard County, TX, Hutchinson County, TX, Navarro County, TX, Giles County, VA, Whatcom County, WA, Outagamie County, WI See: 86 FR 16056
   On April 14, 2021, EPA withdrew the designation of Outagamie County, WI as a nonattainment area See: 86 FR 19576. The Outagamie County area was designated as Attainment/Unclassifiable instead of Nonattainment, effective April 30, 2021.