---
kind: "section"
citation: "40 C.F.R. § 50.5"
title: "40"
number: "50.5"
heading: "National secondary ambient air quality standard for sulfur oxides (sulfur dioxide)."
url: "https://uscodex.org/cfr/40/50.5"
---

# §50.5. National secondary ambient air quality standard for sulfur oxides (sulfur dioxide).

- (a) The level of the 3-hour standard is 0.5 parts per million (ppm), not to be exceeded more than once per calendar year. The 3-hour averages shall be determined from successive nonoverlapping 3-hour blocks starting at midnight each calendar day and shall be rounded to 1 decimal place (fractional parts equal to or greater than 0.05 ppm shall be rounded up).
- (b) Sulfur oxides shall be measured in the ambient air as sulfur dioxide by the reference method described in appendix A of this part or by an equivalent method designated in accordance with [part 53](/cfr/40/part53.md) of this chapter.
- (c) To demonstrate attainment, the second-highest 3-hour average must be based upon hourly data that are at least 75 percent complete in each calendar quarter. A 3-hour block average shall be considered valid only if all three hourly averages for the 3-hour period are available. If only one or two hourly averages are available, but the 3-hour average would exceed the level of the standard when zeros are substituted for the missing values, subject to the rounding rule of [paragraph (a)](#a) of this section, then this shall be considered a valid 3-hour average. In all cases, the 3-hour block average shall be computed as the sum of the hourly averages divided by 3.

## Notes

### Amendments

[61 FR 25580, May 22, 1996]

### Authority

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

### Source

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

### Amendments

[61 FR 25580, May 22, 1996]