5.1 Indoor CO₂ concentrations have been used as indicators of IAQ for many years, involving both appropriate and inappropriate interpretations of indoor CO₂ concentrations (1).5 Appropriate uses include estimating expected levels of occupant comfort in terms of human body odor perception, studying occupancy patterns, investigating the levels of contaminants that are related to occupant activity, and screening for the sufficiency of ventilation rates. Inappropriate uses include estimating outdoor air ventilation rates per person from indoor CO₂ concentrations without properly applying mass balance methods, including verifying the assumptions upon which those methods are based, and using indoor CO₂ concentrations as a comprehensive indicator of IAQ.

5.2 Outdoor air ventilation rates affect airborne contaminant levels in buildings and occupant perceptions of the acceptability of the indoor environment. Minimum rates of outdoor air ventilation are specified in building codes and standards, for example, ASHRAE Standard 62.1 for non-residential buildings. Compliance of outdoor air ventilation rates with relevant codes and standards is often assessed as part of IAQ investigations and energy audits in buildings. Outdoor air ventilation rates of a building depend on the size and distribution of envelope air leakage sites, pressure differences induced by wind and temperature, mechanical system design and operation, and occupant behavior.

5.3 The measurement of CO₂ concentrations has been used to estimate outdoor air ventilation rates. One common approach, referred to in this guide as equilibrium analysis, is based on a steady-state, single-zone mass balance of CO₂ in the building but is too often presented with little or no discussion of its limitations and the assumptions on which it is based. As a result, the technique has been misused and misinterpreted. However, when the assumptions upon which equilibrium analysis is based are valid and the technique is applied properly, it can yield reliable estimates of outdoor air ventilation rates.

5.4 Indoor CO₂ concentrations can be used to determine other aspects of building ventilation when used properly. For example, by applying a mass balance at an air handler, the percent outdoor air intake in the supply airstream can be determined based on the CO₂ concentrations in the supply, return, and outdoor air. This percentage can be multiplied by the supply airflow rate of the air handler to yield the outdoor air intake rate of the air handler. In addition, the decay of indoor CO₂ concentrations can be monitored in a building after the occupants have left to determine the outdoor air change rate of the building.

5.5 Continuous monitoring of indoor CO₂ concentrations can be used to study some aspects of ventilation system performance, the quality of outdoor air, and building occupancy patterns. Continuous monitoring has been used to evaluate overall IAQ. However, at best, indoor CO₂ concentrations can serve as indicators of indoor contaminants emitted at rates that depend on the number of occupants. They do not serve as a comprehensive indicator of IAQ given the many contaminants that do not depend on the number of occupants, such as those entering from outdoors and those emitted by materials, furnishings, cleaning agents, and personal care products.

5.6 Indoor CO₂ regulations and guideline values have been promulgated by multiple organizations, however the justifications for the limits are not necessarily included with the values. They are likely based on either CO₂ as an indicator of ventilation rates, or on the health and performance impacts of CO₂ exposure. There is some evidence of the latter impacts at commonly observed indoor CO₂ concentrations, but it is not consistent and requires further study.

5.7 Measurement of indoor CO₂ concentrations is being promoted as a ventilation indicator in guidance to reduce the risk of infectious aerosol exposure in response to the COVID-19 pandemic. In most cases, this application is based on the equilibrium approach to estimating outdoor air ventilation rates per person, although the technical basis for the values is not always well explained. In other cases, CO₂ is proposed as an indicator of exposure to infectious aerosols emitted by other occupants and the associated risk of disease, although such applications involve numerous assumptions regarding the similarity between the fate and transport of airborne CO₂ and infectious aerosols.

5.8 Health and Performance Impacts of CO₂ Exposure: 

5.8.1 Indoor CO₂ concentrations have been prominent in discussions of ventilation and IAQ since the 18^th century when Antoine Lavoisier suggested that CO₂ build-up rather than oxygen depletion was responsible for “bad air” indoors. About one hundred years later, Max Joseph von Pettenkofer suggested that biological contaminants from human occupants were the source of bad indoor air, not CO₂ (2). More recent research, starting in the 1930s, showed the relationship of indoor CO₂ concentrations to occupant perceptions of odor intensity associated with human bioeffluents as discussed in 7.1 (3).

5.8.2 Indoor CO₂ concentrations are typically on the order of 1000 ppm(v) in non-industrial spaces with excursions up to about 3000 ppm(v) when outdoor air ventilation rates are low relative to the number of occupants. CO₂ is considered to be non-toxic at these levels, with occupational limits of 5000 ppm(v) over a 40 h workweek (4) and 30 000 ppm(v) for short term (15 min) exposures (5). These and other limits are discussed in more detail in 5.9. Toxicological studies have been conducted on the effects of CO₂ exposure in humans and animals, including respiratory, neurologic, reproductive, and cardiovascular impacts, with these studies showing impacts at concentrations of tens of thousands or even hundreds of thousands of ppm(v).

5.8.3 Research results from the 1980s and later found associations between indoor CO₂ levels and occupant health symptoms (in some cases referred to as sick building syndrome symptoms) and school absenteeism. These studies often highlight increases in symptom prevalence and absenteeism at concentrations above 1000 ppm(v). However, these studies did not control for other contaminants that were presumably elevated in association with the reduced per person ventilation rates that led to elevated CO₂ concentrations. Thus, while elevated CO₂ correlated with symptoms and absenteeism, elevated concentrations of other chemicals or biological agents may be the actual cause. More recent studies have examined the impacts of pure CO₂ at concentrations from 600 ppm(v) to 5000 ppm(v) on cognitive performance. For example, some studies have shown reduced cognitive function at concentrations on the order of 1000 ppm(v), while others have not. Based on these inconsistencies, further investigation is merited, including research on potential mechanisms for these impacts.

5.9 Indoor CO₂ Limits in Existing Standards and Building Regulations: 

5.9.1 Several organizations have issued indoor CO₂ concentration limits over the years, though in many cases the rationales for the limits are not documented. In some cases, published limits are based on CO₂ being an indicator of low outdoor air ventilation rates per person and the corresponding exposure to elevated concentrations of other indoor-generated contaminants. An online database of indoor environmental quality guidelines6 contains several national and organizational limits for indoor CO₂ ranging from about 500 ppm(v) above outdoor concentrations to much higher limits for occupational environments. A Canadian residential indoor CO₂ guideline also contains a table of guidelines and standards for indoor CO₂ concentrations (6). Occupational exposure limits for CO₂ in the U.S. are 5000 ppm(v) for an 8 h time weighted average (4) and 30 000 ppm(v) for a 15 min exposure (5). However, these limits are of limited applicability to general indoor environments as they are intended to control exposure to levels that protect worker health, they are not intended to eliminate all effects such as unpleasant odors and irritation and are not based on consideration of the general population that covers a range of age and health status.

5.9.2 More recently, in response to the COVID-19 pandemic, additional CO₂ limits have been promulgated as recommendations and in some cases requirements. For example, the U.S. Centers for Disease Control and Prevention (7), the Federation of European Heating, Ventilation and Air Conditioning Associations (8) in Europe; and Environmental Modelling Group and Scientific Pandemic Insights Group on Behaviours (9) in the United Kingdom have issued recommended indoor CO₂ limits to assess the adequacy of ventilation rates for infection control, with technical rationales for these recommendations discussed in these references to varying degrees of detail. The Belgian Federal Government has issued mandated CO₂ concentration limits (10). The relationship of indoor CO₂ concentrations to infectious aerosol exposure are discussed in 7.3.

5.9.3 In the context of these indoor CO₂ limits, it is important to note that ASHRAE Standard 62.1, Ventilation and Acceptable Indoor Air Quality, contains no such limit and has not since 1989. The 2016 version of the standard did note, in an informative appendix that is not part of the standard, that for an outdoor air ventilation rate of 7.5 L/s per person, the steady-state indoor CO₂ concentration will be about 700 ppm(v) above outdoors and that maintaining indoor concentrations at or below this level “will indicate that a substantial majority of visitors entering a space will be satisfied with respect to human bioeffluents (body odor).” The relationship between indoor CO₂ concentrations and the perception of bioeffluents is discussed in 7.1. While ASHRAE Standard 62.1 does not contain indoor CO₂ limits, it does “specify minimum ventilation rates and other measures intended to provide IAQ that is acceptable to human occupants and that minimizes adverse health effects.”

1.1 This guide describes the relationship of indoor carbon dioxide (CO₂) concentrations to indoor air quality (IAQ) and building ventilation.

1.2 This guide contains background information on the health, comfort, and performance impacts of indoor CO₂ exposure, as well as indoor CO₂ limits contained in various standards and regulations.

1.3 This guide describes the estimation of CO₂ generation rates from people as a function of sex, age, body mass, and level of physical activity.

1.4 This guide describes the relationship of CO₂ to IAQ, including how CO₂ relates to the perception of human body odor, limitations on the application of CO₂ as a metric of IAQ, and the relationship of CO₂ to the risk of infectious aerosol exposure.

1.5 This guide describes how CO₂ concentration measurements can be used to evaluate building ventilation including mass balance analysis to determine the percent outdoor air intake at an air handler, tracer gas decay measurements to estimate whole building air change rates, and use of the constant injection tracer gas technique at equilibrium to estimate whole building air change rates.

1.6 This guide discusses concentration measurement issues, such as calibration and sensor location, and continuous indoor concentration monitoring but does not include specific test methods for either application.

1.7 This guide discusses the use of indoor CO₂ concentrations for demand control ventilation (DCV) but does not contain detailed application guidance.

1.8 Units—The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.9 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.10 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.