Timeline of Ozone National Ambient Air Quality Standards (NAAQS)
| Final Rule/Decision | Primary/Secondary | Indicator1 | Averaging Time | Level2 | Form |
|---|---|---|---|---|---|
| 1971 36 FR 8186 Apr 30, 1971 | Primary and Secondary | Total photochemical oxidants | 1 hour | 0.08 ppm | Not to be exceeded more than one hour per year |
| 1979 44 FR 8202 Feb 8, 1979 | Primary and Secondary | O3 | 1 hour | 0.12 ppm | Attainment is defined when the expected number of days per calendar year, with maximum hourly average concentration greater than 0.12 ppm, is equal to or less than 1 |
| 1993 58 FR 13008 Mar 9, 1993 | EPA decided that revisions to the standards were not warranted at the time | ||||
| 1997 62 FR 38856 Jul 18, 1997 (pdf) (421 KB) | Primary and Secondary | O3 | 8 hours | 0.08 ppm | Annual fourth-highest daily maximum 8-hr concentration, averaged over 3 years |
| 2008 73 FR 16483 Mar 27, 2008 (pdf) (544 KB) | Primary and Secondary | O3 | 8 hours | 0.075 ppm | Annual fourth-highest daily maximum 8-hr concentration, averaged over 3 years |
2015 80 FR 65292 | Primary and Secondary | O3 | 8 hours | 0.070 ppm | Annual fourth-highest daily maximum 8 hour average concentration, averaged over 3 years |
2020 | Primary and secondary standards retained, without revision. | ||||
1. O3 = ozone
2. Units of measure are in parts per million (ppm)