levels……...maintaining CO2 levels below [a specific level] does not mean that a building is in compliance with the standard.

While indoor CO2 concentrations have been shown to be reliable indicators of the acceptability of a space in terms of human body odor, there is little justification for using CO2 as a comprehensive indicator of indoor air quality. ….. Also, many contaminant sources are not associated with occupancy levels, and their concentrations will not be associated with CO2 levels. The analysis of CO2 concentrations can be used to obtain information on building ventilation performance based on a number of tracer gas techniques, but the assumptions associated with these techniques must be understood by the user.

This citation succinctly summarizes many of the misunderstandings and the correct usage of CO2. It can be used as a tracer gas to estimate the outside air intake rate, at a single point-in-time, but only within the guidelines and procedures required in the ASTM Standard D6245-98. The Mass Balance equation in the Appendix of ASHRAE Standard 62 is NOT an endorsement of CO2 measurements as a direct control for ventilation rates. It is shown to allow engineers that under very specific circumstances, it can be used to approximate occupancy and from that data ventilation can be reset to an amount calculated to be required by the indicated occupancy level.

The following assumptions are required by the Mass Balance equation referenced in ASHRAE 62 to make calculated ventilation estimates useful:

1. CO2 measurements should be taken when the space reaches a “steady-state”.
   Interior CO2 concentrations should not fluctuate.
   Outside CO2 concentrations are assumed to be constant in the calculation.
2. CO2 measurements are used in calculation without measurement or sampling error.
   CO2 sensors are assumed not to drift and do not require maintenance or recalibration over time, or between measurements.
3. Human respiration is the same for all building occupants, regardless of: age, sex, size, diet, health, etc.
   Human activity is assumed the same for all building occupants.
   Human activity is assumed to equal that of a seated person.

The conditions described by these assumptions can only occur at a very specific single point-in-time. The corollary is also true – that the assumptions cannot occur in a dynamic, fluid and changing environment.

It is assumed that CO2 is measured with the use of a single, highly accurate instrument and that the calculations needed to usually assume no measurement error. Therefore, any of the possible CO2 methods for the evaluation of ventilation effectiveness cannot be valid, when applied to ventilation control in a dynamic building system, without making numerous and questionable assumptions.

ASHRAE Standard 62-1999

What does the ASHRAE Standard 62-1999 really say, with regard to CO2 measurement, ventilation rate control and IAQ?

Here are some direct quotations with explanations.

FOREWARD (not part of standard)
… Addendum 62f addresses a lack of clarity in ANSI/ASHRAE Standard 62-1989 that has contributed to several misunderstandings regarding the significance of indoor carbon dioxide (CO2) levels. The Standard led many users to conclude that CO2 was itself a

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