Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

Quit Blaming ASHRAE Standard 62.1 for 1000 ppm CO2



Andrew K. Persily


Indoor concentrations of carbon dioxide (CO2) have been widely promoted as metrics of indoor air quality (IAQ) and ventilation, in many cases without a sound understanding or explanation of what they are intended to characterize or adequate discussion of their application and limitations. Many practitioners and researchers use 1800 mg/m3 (roughly 1000 ppmv) as a criteria for defining good IAQ and erroneously cite ASHRAE Standard 62.1 (ASHRAE, 2019) as the source of this value. Standard 62.1 has not contained an indoor CO2 limit for almost 30 years, and no current ASHRAE standard contains an indoor CO2 limit. The CO2 limit was removed from Standard 62.1 based on the confusion that it caused and the fact that it is not a good indicator of ventilation or IAQ. Many papers, presentations and workshops have attempted to clarify the meaning of indoor CO2 concentrations and advocated that they not be used as IAQ or ventilation metrics. However, these efforts have not succeeded in advancing the dialog, and the attribution of the 1800 mg/m3 limit to ASHRAE Standard 62.1 continues. This paper describes what Standard 62.1 says about CO2 now, what it has said in the past, explains the basis for the 1800 mg/m3 value, and stresses any reference value for CO2 to potentially characterize ventilation rates must consider the building type and its occupancy.
Proceedings Title
The 16th Conference of the International Society of Indoor Air Quality & Climate (Indoor Air
Conference Dates
July 20-24, 2020
Conference Location
Seoul, KR


Persily, A. (2020), Quit Blaming ASHRAE Standard 62.1 for 1000 ppm CO2, The 16th Conference of the International Society of Indoor Air Quality & Climate (Indoor Air 2020), Seoul, KR, [online], (Accessed May 18, 2024)


If you have any questions about this publication or are having problems accessing it, please contact

Created November 15, 2020, Updated March 31, 2022