40 CFR § 50.3 - Reference conditions.

---
identifier: "/us/cfr/t40/s50.3"
source: "ecfr"
legal_status: "authoritative_unofficial"
title: "40 CFR § 50.3 - Reference conditions."
title_number: 40
title_name: "Protection of Environment"
section_number: "50.3"
section_name: "Reference conditions."
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: "2026-04-05"
last_updated: "2026-04-05"
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"
---


# 50.3 Reference conditions.

All measurements of air quality that are expressed as mass per unit volume (*e.g.,* micrograms per cubic meter) other than for particulate matter (PM<sub>2.5</sub>) standards contained in §§ 50.7, 50.13, 50.18, and 50.20, and lead standards contained in § 50.16 shall be corrected to a reference temperature of 25 (deg) C and a reference pressure of 760 millimeters of mercury (1,013.2 millibars). Measurements of PM<sub>2.5</sub> for purposes of comparison to the standards contained in §§ 50.7, 50.13, 50.18, and 50.20, and of lead for purposes of comparison to the standards contained in § 50.16 shall be reported based on actual ambient air volume measured at the actual ambient temperature and pressure at the monitoring site during the measurement period.

[89 FR 10365, Dec. 19, 2024]