40 CFR § 50.18 - National primary ambient air quality standards for PM.

---
identifier: "/us/cfr/t40/s50.18"
source: "ecfr"
legal_status: "authoritative_unofficial"
title: "40 CFR § 50.18 - National primary ambient air quality standards for PM."
title_number: 40
title_name: "Protection of Environment"
section_number: "50.18"
section_name: "National primary ambient air quality standards for PM."
chapter_name: "ENVIRONMENTAL PROTECTION AGENCY"
subchapter_number: "C"
subchapter_name: "AIR PROGRAMS"
part_number: "50"
part_name: "NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY STANDARDS"
positive_law: false
currency: "2026-03-24"
last_updated: "2026-03-24"
format_version: "1.1.0"
generator: "[email protected]"
authority: "42 U.S.C. 7401,"
regulatory_source: "36 FR 22384, Nov. 25, 1971, unless otherwise noted."
cfr_part: "50"
---


# 50.18 National primary ambient air quality standards for PM.

(a) The national primary ambient air quality standards for PM<sub>2.5</sub> are 12.0 micrograms per cubic meter (µg/m
<sup>3</sup>) annual arithmetic mean concentration and 35 µg/m
<sup>3</sup> 24-hour average concentration measured in the ambient air as PM<sub>2.5</sub> (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 of this chapter; or

(2) An equivalent method designated in accordance with part 53 of this chapter.

(b) The primary annual PM<sub>2.5</sub> standard is met when the annual arithmetic mean concentration, as determined in accordance with appendix N of this part, is less than or equal to 12.0 µg/m
<sup>3</sup>.

(c) The primary 24-hour PM<sub>2.5</sub> standard is met when the 98th percentile 24-hour concentration, as determined in accordance with appendix N of this part, is less than or equal to 35 µg/m
<sup>3</sup>.

[78 FR 3277, Jan. 15, 2013]