{"data":{"id":"us/40-cfr-51.150","jurisdiction":"us","citation":"40 CFR 51.150","heading":"Classification of regions for episode plans.","body":"(a) This section continues the classification system for episode plans. Each region is classified separately with respect to each of the following pollutants: Sulfur oxides, particulate matter, carbon monoxide, nitrogen dioxide, and ozone.\n(b) Priority I Regions means any area with greater ambient concentrations than the following:\n(1) Sulfur dioxide—100 µg/m 3 (0.04 ppm) annual arithmetic mean; 455 µg/m 3 (0.17 ppm) 24-hour maximum.\n(2) Particulate matter—95 µg/m 3 annual geometric mean; 325 µg/m 3 24-hour maximum.\n(3) Carbon monoxide—55 mg/m 3 (48 ppm) 1-hour maximum; 14 mg/m 3 (12 ppm) 8-hour maximum.\n(4) Nitrogen dioxide—100 µg/m 3 (0.06 ppm) annual arithmetic mean.\n(5) Ozone—195 µg/m 3 (0.10 ppm) 1-hour maximum.\n(c) Priority IA Region means any area which is Priority I primarily because of emissions from a single point source.\n(d) Priority II Region means any area which is not a Priority I region and has ambient concentrations between the following:\n(1) Sulfur Dioxides—60-100 µg/m 3 (0.02-0.04 ppm) annual arithmetic mean; 260-445 µg/m 3 (0.10-0.17 ppm) 24-hour maximum; any concentration above 1,300 µg/m 3 (0.50 ppm) three-hour average.\n(2) Particulate matter—60-95 µg/m 3 annual geometric mean; 150-325 µg/m 3 24-hour maximum.\n(e) In the absence of adequate monitoring data, appropriate models must be used to classify an area under paragraph (b) of this section, consistent with the requirements contained in § 51.112(a).\n(f) Areas which do not meet the above criteria are classified Priority III.","path":["Title 40—Protection of Environment","CHAPTER I—ENVIRONMENTAL PROTECTION AGENCY","SUBCHAPTER C—AIR PROGRAMS","PART 51—REQUIREMENTS FOR PREPARATION, ADOPTION, AND SUBMITTAL OF IMPLEMENTATION PLANS","Subpart H—Prevention of Air Pollution Emergency Episodes"],"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":"6fd0bd8f901bb9a033654c4d709d9f99d16ec9d7ea66176c8be2384fc60c6f31","source_id":"us-cfr","stale":true,"prev":"us/40-cfr-51.126","next":"us/40-cfr-51.151"},"notice":"GroundRules: Original legal text. Not legal advice."}