ASHRAE Journal - February 2021 - 74

IEQ APPLICATIONS
Column

Andrew K. Persily

Don't Blame Standard
62.1 for 1,000 ppm CO2
BY ANDREW K. PERSILY, PH.D., FELLOW/LIFE MEMBER ASHRAE

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 explanation
of what they are intended to characterize or an adequate discussion of the specific
application and any limitations. Many practitioners and researchers use 1,800 mg/m3
(roughly 1,000 ppmv ) as a criteria for defining good IAQ and cite ASHRAE Standard
62.11 as the source of this value. Standard 62.1 has not contained an indoor CO2 limit
since 1989, 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.
Numerous papers, presentations and workshops have
attempted to clarify the significance of indoor CO2 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
1,800 mg/m3 limit to Standard 62.1 continues. This column describes what Standard 62.1 says about CO2 now,
what it has said in the past, explains the basis for the
1,800 mg/m3 value, and stresses that the use of a CO2
reference value to characterize ventilation rates must
consider the building type and its occupancy.

Historical Background

Indoor CO2 concentrations have been discussed in the
context of IAQ and ventilation for centuries. Those discussions have considered the importance of CO2 in relation to bioeffluent perception, its application as an IAQ
metric, CO2 as a contaminant in and of itself, and its use
as a tracer gas to estimate outdoor air ventilation rates.
74

ASHRAE JOURNAL

ashrae.org

FEBRUARY 2021

Despite many attempts to clarify the application of CO2
to IAQ and ventilation,2,3 much confusion has existed
over the past decades and continues today. For example,
numerous statements exist to the effect that a building
has good IAQ because it complies with the 1,000 ppmv
CO2 limit in ASHRAE Standard 62.1. This statement has
multiple problems: we are not able to define " good IAQ " ;
CO2 is not a critically important contaminant in indoor
air; and there is no 1,000 ppmv limit in the standard.
ASHRAE Standard 62-1981 introduced the Indoor Air
Quality Procedure, an alternative, performance-based
design approach in which the ventilation system is
designed to achieve target levels of indoor contaminants. This approach is in contrast to the prescriptive
Ventilation Rate Procedure, in which the design must
meet specific outdoor air ventilation requirements
that are specific to a type of space. As part of the IAQ
Andrew K. Persily, Ph.D., is a division chief in the Engineering Laboratory at the National
Institute of Standards and Technology, Gaithersburg, Md. He is a consultant to SSPC 62.1.

http://ashrae.org

ASHRAE Journal - February 2021

Table of Contents for the Digital Edition of ASHRAE Journal - February 2021