40 CFR § 50.21 - National secondary ambient air quality standards for sulfur oxides (sulfur dioxide).
---
identifier: "/us/cfr/t40/s50.21"
source: "ecfr"
legal_status: "authoritative_unofficial"
title: "40 CFR § 50.21 - National secondary ambient air quality standards for sulfur oxides (sulfur dioxide)."
title_number: 40
title_name: "Protection of Environment"
section_number: "50.21"
section_name: "National secondary 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: "2026-04-05"
last_updated: "2026-04-05"
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"
---
# 50.21 National secondary ambient air quality standards for sulfur oxides (sulfur dioxide).
(a) The level of the annual secondary national ambient air quality standard for oxides of sulfur is 10 parts per billion (ppb), measured in the ambient air as sulfur dioxide (SO<sub>2</sub>) by a reference method based on appendix A-1 and appendix A-2 of this part, or by a Federal Equivalent Method (FEM) designated in accordance with part 53 of this chapter.
(b) The annual secondary standard is met when the 3-year average of the annual SO<sub>2</sub> concentration is less than or equal to 10 ppb, as determined in accordance with appendix T of this part.
[89 FR 105785, Dec. 27, 2024]