---
kind: "section"
citation: "40 C.F.R. § 51.1307"
title: "40"
number: "51.1307"
heading: "Determining eligibility for 1-year attainment date extensions for an 8-hour ozone NAAQS under CAA section 181(a)(5)."
url: "https://uscodex.org/cfr/40/51.1307"
---

# §51.1307. Determining eligibility for 1-year attainment date extensions for an 8-hour ozone NAAQS under CAA section 181(a)(5).

- (a) A nonattainment area will meet the requirement of CAA [section 181(a)(5)(B)](/cfr/40/181.md?p=a-5-B) pertaining to 1-year extensions of the attainment date if:
  - (1) For the first 1-year extension, the area's 4th highest daily maximum 8-hour average in the attainment year is no greater than the level of that NAAQS.
  - (2) For the second 1-year extension, the area's 4th highest daily maximum 8-hour value, averaged over both the original attainment year and the first extension year, is no greater than the level of that NAAQS.
- (b) For purposes of [paragraph (a)(1)](#a-1) of this section, the area's 4th highest daily maximum 8-hour average for a year shall be from the monitor with the highest 4th highest daily maximum 8-hour average for that year of all the monitors that represent that area.
- (c) For purposes of [paragraph (a)(2)](#a-2) of this section, the area's 4th highest daily maximum 8-hour value, averaged over both the original attainment year and the first extension year, shall be from the monitor in each year with the highest 4th highest daily maximum 8-hour average of all monitors that represent that area.

## Notes

### Amendments

[83 FR 63033, Dec. 6, 2018]

### Source

Source: 83 FR 10382, Mar. 9, 2018, unless otherwise noted.

### Authority

Authority: 23 U.S.C. 101; 42 U.S.C. 7401-7671q.

### Source

Source: 36 FR 22398, Nov. 25, 1971, unless otherwise noted.

### Amendments

[83 FR 63033, Dec. 6, 2018]