---
kind: "section"
citation: "40 C.F.R. § 50.20"
title: "40"
number: "50.20"
heading: "National primary ambient air quality standards for PM 2.5 ."
url: "https://uscodex.org/cfr/40/50.20"
---

# §50.20. National primary ambient air quality standards for PM 2.5 .

- (a) The national primary ambient air quality standards for PM2.5 are 9.0 micrograms per cubic meter (µg/m 3) annual arithmetic mean concentration and 35 µg/m 3 24-hour average concentration measured in the ambient air as PM2.5 (particles with an aerodynamic diameter less than or equal to a nominal 2.5 micrometers) by either:
  - (1) A reference method based on appendix L to this part and designated in accordance with [part 53](/cfr/40/part53.md) of this chapter; or
  - (2) **An equivalent method designated in accordance with part 53 of this chapter.**
- (b) The primary annual PM2.5 standard is met when the annual arithmetic mean concentration, as determined in accordance with appendix N to this part, is less than or equal to 9.0 µg/m 3.
- (c) The primary 24-hour PM2.5 standard is met when the 98th percentile 24-hour concentration, as determined in accordance with appendix N to this part, is less than or equal to 35 µg/m 3.

## Notes

### Amendments

[89 FR 16380, Mar. 6, 2024]

### Authority

Authority: 42 U.S.C. 7401, et seq.

### Source

Source: 36 FR 22384, Nov. 25, 1971, unless otherwise noted.

### Amendments

[89 FR 16380, Mar. 6, 2024]