40 CFR § 51.151 - Significant harm levels.

---
identifier: "/us/cfr/t40/s51.151"
source: "ecfr"
legal_status: "authoritative_unofficial"
title: "40 CFR § 51.151 - Significant harm levels."
title_number: 40
title_name: "Protection of Environment"
section_number: "51.151"
section_name: "Significant harm levels."
chapter_name: "ENVIRONMENTAL PROTECTION AGENCY"
subchapter_number: "C"
subchapter_name: "AIR PROGRAMS"
part_number: "51"
part_name: "REQUIREMENTS FOR PREPARATION, ADOPTION, AND SUBMITTAL OF IMPLEMENTATION PLANS"
positive_law: false
currency: "2026-04-05"
last_updated: "2026-04-05"
format_version: "1.1.0"
generator: "[email protected]"
authority: "23 U.S.C. 101; 42 U.S.C. 7401-7671q."
regulatory_source: "36 FR 22398, Nov. 25, 1971, unless otherwise noted."
cfr_part: "51"
---


# 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
<sup>3</sup> (1.0 ppm) 24-hour average.

PM<sub>10</sub>—600 micrograms/cubic meter; 24-hour average.

*Carbon monoxide*—57.5 mg/m
<sup>3</sup> (50 ppm) 8-hour average; 86.3 mg/m
<sup>3</sup> (75 ppm) 4-hour average; 144 mg/m
<sup>3</sup> (125 ppm) 1-hour average.

*Ozone*—1,200 ug/m
<sup>3</sup> (0.6 ppm) 2-hour average.

*Nitrogen dioxide*—3.750 ug/m
<sup>3</sup> (2.0 ppm) 1-hour average; 938 ug/m
<sup>3</sup> (0.5 ppm) 24-hour average.

[51 FR 40668, Nov. 7, 1986, as amended at 52 FR 24713, July 1, 1987]