Each plan for a Priority I region must include a contingency plan which must, as a mimimum, 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 [micro]g/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. [51 FR 40668, Nov. 7, 1986, as amended at 52 FR 24713, July 1, 1987]