Timeline of Sulfur Dioxide National Ambient Air Quality Standards (NAAQS)
| Final Rule/Decision | Primary/ Secondary | Indicator (1) | Averaging Time | Level (2) | Form | |
|---|---|---|---|---|---|---|
|
1971 36 FR 8186 |
Primary |
SO2 |
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 (3) |
0.02 ppm |
Annual arithmetic average |
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|
1973 |
Secondary |
Secondary 3-hour SO2 standard retained, without revision; secondary annual SO2 standard revoked. |
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|
1996 61 FR 25566 |
Primary |
Existing primary SO2 standards retained, without revision. |
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|
2010 |
Primary |
SO2 |
1-hour |
75 ppb |
99th percentile, averaged over 3 years (5) |
|
|
Primary annual and 24-hour SO2 standards revoked. |
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|
2012 77 FR 20218 |
Secondary |
Existing secondary SO2 standard (3-hour average) retained, without revision. |
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|
2019 |
Primary |
Existing primary SO2 standard retained, without revision. |
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|
2024 89 FR 105692 Dec 27, 2024 (pdf) (1,510 KB) |
Secondary | Secondary SO2 standard revised to be an annual standard. | ||||
| SO2 | Annual | 10 ppb | Averaged over 3 years | |||
(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 established in 2010 is a primary standard.
(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.
Learn more about setting and reviewing standards to control SO2 pollution.