Please Don’t Blame Standard 62.1 for 1000 ppm CO2

Indoor concentrations of carbon dioxide (CO 2 ) have been widely promoted as metrics of indoor air quality (IAQ) and ventilation, in many cases without a sound explanation of what they are intended to characterize or an adequate discussion of the specific application and any limitations. Many practitioners and researchers use 1800 mg/m 3 (roughly 1000 ppm v ) as a criteria for defining good IAQ and cite ASHRAE Standard 62.1 (ASHRAE, 2019) as the source of this value. Standard 62.1 has not contained an indoor CO 2 limit for almost 30 years, and no current ASHRAE standard contains an indoor CO 2 limit. The CO 2 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. Numerous papers, presentations and workshops have attempted to clarify the significance of indoor CO 2 concentrations and even advocated that they not be used as IAQ or ventilation metrics. However, these efforts have not ended the confusion, and the attribution of a 1800 mg/m 3 limit to Standard 62.1 continues. This paper describes what Standard 62.1 says about CO 2 now, what it has said in the past, explains the basis for the 1800 mg/m 3 value, and stresses that the use of a CO 2 reference value to characterize ventilation rates must consider the building type and its occupancy.