Sulfur Dioxide (SO₂) Primary Standards -
Table of Historical SO₂ NAAQS

History of the National Ambient Air Quality Standards for Oxides of Sulfur
During the Period 1971-2010

Final Rule/Decision	Primary/ Secondary	Indicator ⁽¹⁾	Averaging Time	Level ⁽²⁾	Form
1971 36 FR 8186 Apr 30, 1971	Primary	SO₂	24-Hour	0.14 ppm	Not to be exceeded more than once per year
	Primary		Annual	0.03 ppm	Annual arithmetic average
	Secondary		3-Hour	0.5 ppm	Not to be exceeded more than once per year
	Secondary		Annual ⁽³⁾	0.02 ppm	Annual arithmetic average
1973 38 FR 25678 Sept 14, 1973	Secondary	Secondary 3-hour SO₂ standard retained, without revision; secondary annual SO₂ standard revoked.
1996 61 FR 25566 May 22, 1996	Primary	Existing primary SO₂ standards retained, without revision.
2010 75 FR 35520 Jun 22, 2010 ⁽⁴⁾	Primary	SO₂	1-hour	75 ppb	99th percentile, averaged over 3 years ⁽⁵⁾
2010 75 FR 35520 Jun 22, 2010 ⁽⁴⁾	Primary	Primary annual and 24-hour SO₂ standards revoked.

⁽¹⁾ SO₂ = sulfur dioxide

⁽²⁾ Units of measure are in parts per million (ppm) and parts per billion (ppb).

⁽³⁾ The 1971 final rule also included a secondary 24-hour SO₂ standard of 0.1 ppm, maximum 24-hour concentration not to be exceeded more than once per year, as a guide to be used in assessing implementation plans to achieve the annual standard.

⁽⁴⁾ The 1-hour SO₂ standard added in 2010 is a primary standard. The revision of the SO₂ NAAQS in 2010 did not address the secondary standard, which remains a 3-hour standard with a level of 0.5 ppm.

⁽⁵⁾ The form of the 1-hour standard is the 3-year average of the 99th percentile of the yearly distribution of 1-hour daily maximum SO₂ concentrations.

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Sulfur Dioxide (SO₂) Primary Standards -
Table of Historical SO₂ NAAQS