---
kind: "section"
citation: "40 C.F.R. § 50.19"
title: "40"
number: "50.19"
heading: "National primary and secondary ambient air quality standards for ozone."
url: "https://uscodex.org/cfr/40/50.19"
---

# §50.19. National primary and secondary ambient air quality standards for ozone.

- (a) The level of the national 8-hour primary ambient air quality standard for ozone (O3) is 0.070 parts per million (ppm), daily maximum 8-hour average, 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 or an equivalent method designated in accordance with [part 53](/cfr/40/part53.md) of this chapter.
- (b) The 8-hour primary O3 ambient air quality standard is met at an ambient air quality monitoring site when the 3-year average of the annual fourth-highest daily maximum 8-hour average O3 concentration is less than or equal to 0.070 ppm, as determined in accordance with appendix U to this part.
- (c) The level of the national secondary ambient air quality standard for O3 is 0.070 ppm, daily maximum 8-hour average, 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 or an equivalent method designated in accordance with [part 53](/cfr/40/part53.md) of this chapter.
- (d) The 8-hour secondary O3 ambient air quality standard is met at an ambient air quality monitoring site when the 3-year average of the annual fourth-highest daily maximum 8-hour average O3 concentration is less than or equal to 0.070 ppm, as determined in accordance with appendix U to this part.

## Notes

### Amendments

[80 FR 65452, Oct. 26, 2015]

### Authority

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

### Source

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

### Amendments

[80 FR 65452, Oct. 26, 2015]