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SUMMARY OF ASHRAE’S POSITION ON CARBON DIOXIDE(CO2) LEVELS IN SPACES

Stephen Petty, P.E., C.I.H.

Purpose of the Summary Statement:

It is widely reported by the technical community involved in indoor air evaluations thatthe American Society of Heating, Refrigeration and Air Conditioning Engineers(ASHRAE) has a standard of 1,000 ppm CO2  for indoor spaces. The Standard oftencited is ANSI/ASHRAE 62-1989 “Ventilation for Acceptable Indoor Air Quality” (whichhas since been replaced by ANSI/ASHRAE 62-1999). However, this interpretation is

incorrect.

The purposes of this Summary Statement on Carbon Dioxide Levels is to providebackground and information on the basis of the “1,000 ppm CO2” guideline value andhow it was intended to be used regarding indoor air quality. As will be discussed below,this “1,000 ppm CO2” value is not contained in the latest ANSI/ASHRAE 62-1999standard.

Background:

One of the best papers addressing this issue was prepared by Mike Schell and Dan Int-Hout entitled “Demand Control Ventilation Using CO2” published in the February, 2001,

 ASHRAE Journal (copy attached as Attachment A). This article points out that CO2

has long been used as a basis for ventilation (providing fresh outdoor air to indoorspaces) design and control. CO2  is a natural product of human respiration whose ratecan be predicted based on an occupant’s age and activity level. Beginning as early as1916 (Mechanical Engineer’s Handbook   by McGraw-Hill) and found in the New YorkCity Building Code of 1929, CO2 of 800 to 1,000 ppm and 1,000 ppm respectively wererecommended. However, the key point is that CO2   levels are good predictors orsurrogates for human emitted bioeffluents (i.e., odors) that are considered undesirablefor the overall human comfort inside conditioned spaces.  Thus CO2  is a surrogate for

levels of other bioeffluents that cause odors that are likely to be viewed as unacceptableby others in the space, not because of their presence as a direct health hazard.

It is helpful to review the basis for the 1,000 ppm CO2 as well as the language in the ANSI/ASHRAE 62-1989 standard and Interpretation Documents from ASHRAE on thismatter.

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The 1,000 ppm guideline value for CO2  used in the 1989 standard was based on anassumed ventilation air (outdoor air) rate of 15 CFM/person and an outdoor baselineCO2 concentration of 300 ppm. (Note that the 1999 standard set a differential CO2 levelof 700 ppm along with minimum ventilation rates for given spaces rather than anabsolute value like 1,000 ppm. This change to the standard occurred because

background CO2 levels are closer to 400 ppm than 300 ppm resulting in an indoor levelof 1,100 ppm CO2 using the same 15 CFM/person as was used in the 1989 standard.Thus the current standard would have a baseline above 1,000 ppm CO2. Moreover, aswill be discussed, this absolute value led to confusion in the design, and health andsafety communities.)

Specific language contained in the 1989 standard are reproduced below:

Section 6.1.3: “Human occupants produce carbon dioxide, water vapor, particulates,biological aerosols, and other contaminants. Carbon dioxideconcentration has been widely used as an indicator of indoor air

quality. Comfort (odor) criteria are likely to be satisfied if the ventilationrate is set so that 1000 ppm CO2  is not exceeded. In the event CO2 iscontrolled by any method other than dilution, the effects of possibleelevation of other contaminants must be considered.”

Section 6.2.1: Repeats the Section 6.1.3 language of the standard.

Table 3: “Guidelines for Selected Air Contaminants of Indoor Origin”

Contaminant Concentration ppm Exposure Time Comments

Carbon Dioxide 1.8 g/m3  1000a  Continuous See App. D

a This level is not considered a health risk but is a surrogate for human comfort (odor).See Section 6.1.3 and Appendix D.

This last statement clearly defines the role of CO2 in the standard.

 ASHRAE also produced a document entitled “Interpretations for ASHRAE Standard 62-1989 Ventilation for Acceptable Indoor Air Quality” that provided specific interpretationsof this standard. Two questions, and their responses, (see Attachment B) are directlyrelevant to this current topic and confirm the last statement just discussed.

Q: June 26, 1995 – The carbon dioxide level of 1000 ppm noted in 6.1.3 and 6.2including Table 3 appears to be provided as a recommended guideline ratherthan a mandatory requirement. Can the carbon dioxide level in a space everexceed the referenced value of 1000 ppm, and still remain in compliance with thestandard?

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 A: Yes. The CO2 level of 1000 is a guideline for comfort acceptability, not a ceilingvalue for air quality.

 A similar series of questions on the topic were responded to on January 29, 1995:

Ms. Paolini, Manager of Health and Safety notes that….It is unclear what is meant bythe clause, “ventilation rate is set so the 1000 ppm CO2 is not exceeded.”

Q1: Is the 1000 ppm CO2 a ceiling value or a time weighted average value?

 A1: The reference to 1000 ppm CO2 in Section 6.1.3 is only as a point of information.This is not a requirement of ASHRAE 62-1989. Since it is not a requirement it isneither a ceiling nor a time weighted average value. Rather, it can be considereda target concentration level. Since comfort (odor) criteria are likely to be satisfiedwhen the CO2 does not exceed 1000 ppm the converse is also likely to be true,i.e., when the CO2  level exceeds 1000 ppm, the comfort (odor) criteria may not

be satisfied.

Q2: If it is a time weighted average value, how are the CO2  test results to becalculated and weighted?

 A2: Moot because of Answer 1.

Q3: Would CO2 levels measured only during room occupancy be used or CO2 levelsmeasured throughout the time period of ventilation system operation?

 A3: CO2 levels should be measured during the time of occupancy. This is defined forthe classroom as the time between initial occupancy in the morning anddismissal time for the students.

 Again, these questions and answers suggest that the CO2  level of 1,000 ppm is aguideline (surrogate for odors), not a standard.

One should note that the 1999 Standard has removed all mention of the 1,000 ppmvalue from the Standard.

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Summary and Conclusions:

• The current ASHRAE Standard “Ventilation for Acceptable Indoor Air Quality”(ANSI/ASHRAE 62-1999) does not reference the term “1,000 ppm CO2.”

• The former ASHRAE Standard “Ventilation for Acceptable Indoor Air Quality”

(ANSI/ASHRAE 62-1989) references the term “1,000 ppm CO2 as a surrogate forwhere human bioeffluents (odors) may be at levels not acceptable for humancomfort. Further, this value of 1,000 is a guideline value only and not considereda regulated standard.

• Interpretation document questions and answers confirm this concept of the CO2 concentration of 1,000 ppm as a guideline level to be used as a surrogate forodor causing compounds from human activity that may not be acceptable forhuman comfort.

If you have any questions or comments regarding this document, please contact theauthor at:

Energy & Environmental Solutions, Inc.84 N. High Street, Suite 2BDublin, OH 43017

www.eesinc.cc 

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ATTACHMENT A

Demand Control Ventilation Using CO2 

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ATTACHMENT B

Portions of ASHRAE Standard 62-1989 Interpretation

Document

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