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 | |||
| Secondary | 3-Hour | 0.5 ppm | Not to be exceeded more than once per year | ||
| Annual (3) | 0.02 ppm | Annual arithmetic average | |||
1973 | Secondary | Secondary 3-hour SO2 standard retained, without revision; secondary annual SO2 standard revoked. | |||
1993 | Secondary | Existing secondary SO2 standard retained, without revision. | |||
1996 61 FR 25566 | Primary | Existing primary SO2 standards retained, without revision. | |||
2010 | Primary | SO2 | 1-hour | 75 ppb | 99th percentile, averaged over 3 years (5) |
| Primary annual and 24-hour SO2 standards revoked. | |||||
2012 77 FR 20218 | Secondary | Existing secondary SO2 standard (3-hour average) retained, without revision. | |||
2019 | Primary | Existing primary SO2 standard retained, without revision. | |||
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.