{"data":{"id":"us/40-cfr-50.17","jurisdiction":"us","citation":"40 CFR 50.17","heading":"National primary ambient air quality standards for sulfur oxides (sulfur dioxide).","body":"(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).\n(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.\n(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.","path":["Title 40—Protection of Environment","CHAPTER I—ENVIRONMENTAL PROTECTION AGENCY","SUBCHAPTER C—AIR PROGRAMS","PART 50—NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY STANDARDS"],"source_url":"https://www.ecfr.gov/api/versioner/v1/full/2026-08-25/title-40.xml","current_through":"2026-08-25","vintage":"","retrieved_at":"2026-08-27T02:26:04Z","sha256":"c76aa2c0bc650390d753d8494940b7b740261d3aae98dff03815b55d059cf06c","source_id":"us-cfr","stale":true,"prev":"us/40-cfr-50.16","next":"us/40-cfr-50.18"},"notice":"GroundRules: Original legal text. Not legal advice."}