40 CFR § 51.151
Significant harm levels
June 9, 2020
Each plan for a Priority I region must include a contingency plan which must, as a minimum, provide for taking action necessary to prevent ambient pollutant concentrations at any location in such region from reaching the following levels:
Sulfur dioxide—2.620 µg/m3 (1.0 ppm) 24-hour average.
PM10—600 micrograms/cubic meter; 24-hour average.
Carbon monoxide—57.5 mg/m3 (50 ppm) 8-hour average; 86.3 mg/m3 (75 ppm) 4-hour average; 144 mg/m3 (125 ppm) 1-hour average.
Ozone—1,200 ug/m3 (0.6 ppm) 2-hour average.
Nitrogen dioxide—3.750 ug/m3 (2.0 ppm) 1-hour average; 938 ug/m3 (0.5 ppm) 24-hour average.
[51 FR 40668, Nov. 7, 1986, as amended at 52 FR 24713, July 1, 1987]
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