---
kind: "section"
citation: "40 C.F.R. § 51.150"
title: "40"
number: "51.150"
heading: "Classification of regions for episode plans."
url: "https://uscodex.org/cfr/40/51.150"
---

# §51.150. Classification of regions for episode plans.

- (a) **This section continues the classification system for episode plans.** Each region is classified separately with respect to each of the following pollutants: Sulfur oxides, particulate matter, carbon monoxide, nitrogen dioxide, and ozone.
- (b) Priority I Regions means any area with greater ambient concentrations than the following:
  - (1) **Sulfur dioxide—** 100 µg/m 3 (0.04 ppm) annual arithmetic mean; 455 µg/m 3 (0.17 ppm) 24-hour maximum.
  - (2) **Particulate matter—** 95 µg/m 3 annual geometric mean; 325 µg/m 3 24-hour maximum.
  - (3) **Carbon monoxide—** 55 mg/m 3 (48 ppm) 1-hour maximum; 14 mg/m 3 (12 ppm) 8-hour maximum.
  - (4) **Nitrogen dioxide—** 100 µg/m 3 (0.06 ppm) annual arithmetic mean.
  - (5) **Ozone—** 195 µg/m 3 (0.10 ppm) 1-hour maximum.
- (c) Priority IA Region means any area which is Priority I primarily because of emissions from a single point source.
- (d) Priority II Region means any area which is not a Priority I region and has ambient concentrations between the following:
  - (1) **Sulfur Dioxides—** 60-100 µg/m 3 (0.02-0.04 ppm) annual arithmetic mean; 260-445 µg/m 3 (0.10-0.17 ppm) 24-hour maximum; any concentration above 1,300 µg/m 3 (0.50 ppm) three-hour average.
  - (2) **Particulate matter—** 60-95 µg/m 3 annual geometric mean; 150-325 µg/m 3 24-hour maximum.
- (e) In the absence of adequate monitoring data, appropriate models must be used to classify an area under [paragraph (b)](#b) of this section, consistent with the requirements contained in [§ 51.112(a)](/cfr/40/51.112.md?p=a).
- (f) **Areas which do not meet the above criteria are classified Priority III.**

## Notes

### Amendments

[51 FR 40668, Nov. 7, 1986, as amended at 58 FR 38822, July 20, 1993]

### Source

Source: 51 FR 40668, Nov. 7, 1986, unless otherwise noted.

### Authority

Authority: 23 U.S.C. 101; 42 U.S.C. 7401-7671q.

### Source

Source: 36 FR 22398, Nov. 25, 1971, unless otherwise noted.

### Amendments

[51 FR 40668, Nov. 7, 1986, as amended at 58 FR 38822, July 20, 1993]