Code of Federal Regulations Title 40. Protection of Environment § 40.51.151 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/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.