---
kind: "section"
citation: "40 C.F.R. § 50.17"
title: "40"
number: "50.17"
heading: "National primary ambient air quality standards for sulfur oxides (sulfur dioxide)."
url: "https://uscodex.org/cfr/40/50.17"
---

# §50.17. National primary ambient air quality standards for sulfur oxides (sulfur dioxide).

- (a) The level of the national primary 1-hour annual ambient air quality standard for oxides of sulfur is 75 parts per billion (ppb, which is 1 part in 1,000,000,000), measured in the ambient air as sulfur dioxide (SO2).
- (b) The 1-hour primary standard is met at an ambient air quality monitoring site when the three-year average of the annual (99th percentile) of the daily maximum 1-hour average concentrations is less than or equal to 75 ppb, as determined in accordance with appendix T of this part.
- (c) The level of the standard shall be measured by a reference method based on appendix A or A-1 of this part, or by a Federal Equivalent Method (FEM) designated in accordance with [part 53](/cfr/40/part53.md) of this chapter.

## Notes

### Amendments

[75 FR 35592, June 22, 2010]

### Authority

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

### Source

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

### Amendments

[75 FR 35592, June 22, 2010]