40 CFR 51.151 — Significant harm levels.

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/m 3 (1.0 ppm) 24-hour average. PM10—600 micrograms/cubic meter; 24-hour average. Carbon monoxide—57.5 mg/m 3 (50 ppm) 8-hour average; 86.3 mg/m 3 (75 ppm) 4-hour average; 144 mg/m 3 (125 ppm) 1-hour average. Ozone—1,200 ug/m 3 (0.6 ppm) 2-hour average. Nitrogen dioxide—3.750 ug/m 3 (2.0 ppm) 1-hour average; 938 ug/m 3 (0.5 ppm) 24-hour average.