40 CFR § 50.17 - National primary ambient air quality standards for sulfur oxides (sulfur dioxide).
---
identifier: "/us/cfr/t40/s50.17"
source: "ecfr"
legal_status: "authoritative_unofficial"
title: "40 CFR § 50.17 - National primary ambient air quality standards for sulfur oxides (sulfur dioxide)."
title_number: 40
title_name: "Protection of Environment"
section_number: "50.17"
section_name: "National primary ambient air quality standards for sulfur oxides (sulfur dioxide)."
chapter_name: "ENVIRONMENTAL PROTECTION AGENCY"
subchapter_number: "C"
subchapter_name: "AIR PROGRAMS"
part_number: "50"
part_name: "NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY STANDARDS"
positive_law: false
currency: "2026-03-24"
last_updated: "2026-03-24"
format_version: "1.1.0"
generator: "[email protected]"
authority: "42 U.S.C. 7401,"
regulatory_source: "36 FR 22384, Nov. 25, 1971, unless otherwise noted."
cfr_part: "50"
---
# 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 (SO<sub>2</sub>).
(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.
[75 FR 35592, June 22, 2010]