#
Sulfur Dioxide (SO_{2}) Primary Standards -

Table of Historical SO_{2} 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 |
Primary |
SO |
24-Hour |
0.14 ppm |
Not to be exceeded more than once per year |

Annual |
0.03 ppm |
Annual arithmetic average |
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Secondary |
3-Hour |
0.5 ppm |
Not to be exceeded more than once per year |
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Annual |
0.02 ppm |
Annual arithmetic average |
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1973 38 FR 25678 |
Secondary |
Secondary 3-hour SO |
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1996 61 FR 25566 |
Primary |
Existing primary SO |
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2010 75 FR 35520 |
Primary |
SO |
1-hour |
75 ppb |
99th percentile, averaged over 3 years |

Primary annual and 24-hour SO |

^{(1)} SO_{2} = 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 SO_{2} 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 SO_{2} standard added in 2010 is a primary standard. The revision of the SO_{2} 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 SO_{2} concentrations.