{"data":{"id":"us/40-cfr-50.20","jurisdiction":"us","citation":"40 CFR 50.20","heading":"National primary ambient air quality standards for PM2.5.","body":"(a) The national primary ambient air quality standards for PM2.5 are 9.0 micrograms per cubic meter (µg/m 3) annual arithmetic mean concentration and 35 µg/m 3 24-hour average concentration measured in the ambient air as PM2.5 (particles with an aerodynamic diameter less than or equal to a nominal 2.5 micrometers) by either:\n(1) A reference method based on appendix L to this part and designated in accordance with part 53 of this chapter; or\n(2) An equivalent method designated in accordance with part 53 of this chapter.\n(b) The primary annual PM2.5 standard is met when the annual arithmetic mean concentration, as determined in accordance with appendix N to this part, is less than or equal to 9.0 µg/m 3.\n(c) The primary 24-hour PM2.5 standard is met when the 98th percentile 24-hour concentration, as determined in accordance with appendix N to this part, is less than or equal to 35 µg/m 3.","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":"839269a2c7405da592be00564b742607ebffadbcbb7e47a75abf3b69bef1bf1d","source_id":"us-cfr","stale":true,"prev":"us/40-cfr-50.19","next":"us/40-cfr-50.21"},"notice":"GroundRules: Original legal text. Not legal advice."}