---
kind: "section"
citation: "40 C.F.R. § 50.9"
title: "40"
number: "50.9"
heading: "National 1-hour primary and secondary ambient air quality standards for ozone."
url: "https://uscodex.org/cfr/40/50.9"
---

# §50.9. National 1-hour primary and secondary ambient air quality standards for ozone.

- (a) The level of the national 1-hour primary and secondary ambient air quality standards for ozone measured by a reference method based on appendix D to this part and designated in accordance with [part 53](/cfr/40/part53.md) of this chapter, is 0.12 parts per million (235 µg/m 3). The standard is attained when the expected number of days per calendar year with maximum hourly average concentrations above 0.12 parts per million (235 µg/m 3) is equal to or less than 1, as determined by appendix H to this part.
- (b) The 1-hour standards set forth in this section will remain applicable to all areas notwithstanding the promulgation of 8-hour ozone standards under [§ 50.10](/cfr/40/50.10.md). The 1-hour NAAQS set forth in [paragraph (a)](#a) of this section will no longer apply to an area one year after the effective date of the designation of that area for the 8-hour ozone NAAQS pursuant to [section 107](/cfr/40/107.md) of the Clean Air Act. Area designations and classifications with respect to the 1-hour standards are codified in [40 CFR part 81](/cfr/40/part81.md).

## Notes

### Amendments

[62 FR 38894, July 18, 1997, as amended at 65 FR 45200, July 20, 2000; 68 FR 38163, June 26, 2003, 69 FR 23996, Apr. 30, 2004; 77 FR 28441, May 14, 2012]

### Authority

Authority: 42 U.S.C. 7401, et seq.

### Source

Source: 36 FR 22384, Nov. 25, 1971, unless otherwise noted.

### Amendments

[62 FR 38894, July 18, 1997, as amended at 65 FR 45200, July 20, 2000; 68 FR 38163, June 26, 2003, 69 FR 23996, Apr. 30, 2004; 77 FR 28441, May 14, 2012]