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Ventilation for Acceptable Indoor Air Quality

ASHRAE Standard 62 is and has been a consensus standard that provides designers of buildings and mechanical ventilation systems guidance on ventilation for acceptable indoor air quality. The new standard contains a number of significant revisions to the 2001 standard. There have been a number of changes to the standard over the years. One example is the use of carbon dioxide levels as an indicator of indoor air quality.

Beginning with the 1981 standard, and picking up momentum with the 1989 standard, indoor air quality professionals used the standard to evaluate building air quality and make recommendations to building owners and managers. These recommendations often involved increasing the amount of outdoor air entering the building. This recommendation was usually based upon a measurement of the indoor carbon dioxide concentration. Carbon dioxide (CO2) is generated by building occupants and has been used to indicate whether enough outside air was entering the building. Outside air dilutes the concentration of carbon dioxide and reduces it or maintains it at or below a certain level. The level established in 1989 by ASHRAE in Standard 62 was 1000 parts per million (ppm) or 0.1%. This level was never intended to indicate that 1000 ppm of CO2 was hazardous; it was used as an indicator that other air pollutants and odors that were difficult to measure could be at unacceptable levels. It was also an easy to quote number that was easier to use than measuring the actual outdoor air ventilation rates and comparing them to the outdoor air requirements contained in the standard.

In the 1999 revision of the standard, due to “misunderstandings regarding the significance of indoor carbon dioxide (CO2) levels”, the 1000 ppm recommended limit was dropped. Replacing it was language stating that odors could be adequately controlled if CO2 levels could be maintained below a concentration that was 700 ppm above the outdoor concentration. There was also an Appendix D that explained the rationale for 700 ppm above the outdoor concentration. Although not as strongly worded in the standard, a CO2 concentration that was 700 ppm above the outside concentration became the new recommended limit for many indoor air quality professionals. This statement regarding 700 ppm was maintained in the 2001 Standard.

The 2004 Standard eliminates this statement. The only mention of CO2 within the standard is in a “Note:” in paragraph 6.2.7 that mentions CO2 monitoring as an example of a method of determining occupant variability. The former Appendix D has been retained as Appendix C, but has a statement above it that emphasizes that it is not a part of the standard.

The changes to the use of carbon dioxide as an indicator of indoor air quality are by no means the most significant change to the new standard but serve as an example of how criteria can change over time. The new ANSI/ASHRAE Standard 62.1-2004 Ventilation for Acceptable Indoor Air Quality incorporates a number of significant changes to the previous standard – 62-2001.

This new standard incorporates 17 addenda that delete and/or replace many of the sections of the previous standard. In fact, there is relatively little that has not been changed. Part 2 of this article will cover those changes in depth.