{"data":{"id":"us/40-cfr-50.19","jurisdiction":"us","citation":"40 CFR 50.19","heading":"National primary and secondary ambient air quality standards for ozone.","body":"(a) The level of the national 8-hour primary ambient air quality standard for ozone (O3) is 0.070 parts per million (ppm), daily maximum 8-hour average, measured by a reference method based on appendix D to this part and designated in accordance with part 53 of this chapter or an equivalent method designated in accordance with part 53 of this chapter.\n(b) The 8-hour primary O3 ambient air quality standard is met at an ambient air quality monitoring site when the 3-year average of the annual fourth-highest daily maximum 8-hour average O3 concentration is less than or equal to 0.070 ppm, as determined in accordance with appendix U to this part.\n(c) The level of the national secondary ambient air quality standard for O3 is 0.070 ppm, daily maximum 8-hour average, measured by a reference method based on appendix D to this part and designated in accordance with part 53 of this chapter or an equivalent method designated in accordance with part 53 of this chapter.\n(d) The 8-hour secondary O3 ambient air quality standard is met at an ambient air quality monitoring site when the 3-year average of the annual fourth-highest daily maximum 8-hour average O3 concentration is less than or equal to 0.070 ppm, as determined in accordance with appendix U to this part.","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":"0dc29c1e9a363174cee58d6e997ddf4370d85a240993a29f0765753584a8a0ef","source_id":"us-cfr","stale":true,"prev":"us/40-cfr-50.18","next":"us/40-cfr-50.20"},"notice":"GroundRules: Original legal text. Not legal advice."}