On March 1, 2018, EPA released a final rule defining nonattainment area classifications under the 2015 ozone standard, along with attainment deadlines for each classification. The rule finalizes the classifications and deadlines that were originally proposed by the Obama administration in a proposed rule issued on November 17, 2016. (81 Fed. Reg. 81,276). According to Section 181(a)(1) of the Clean Air Act, nonattainment areas must be classified at the time of designation, so this rulemaking clears the way for EPA to issue final designations for the 2015 standard. The air quality thresholds for each classification and the associated attainment deadlines are listed in the chart below. The final rule has not yet been published in the Federal Register.
| CLASSIFICATION | THRESHOLD | MAXIMUM ATTAINMENT DATE |
| Marginal | 71 ppb up to 81 ppb | 3 years |
| Moderate | 81 ppb up to 93 ppb | 6 years |
| Serious | 93 ppb up to 105 ppb | 9 years |
| Severe | 105 ppb up to 163 ppb | 15 years (or 17 years) |
| Extreme | 163 ppb | 20 years |
* from effective date of designation