Code of Federal Regulations Title 40. Protection of Environment § 40.50.17 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 of this chapter.