Significance and Use
Indoor CO2 concentrations have been described and used by some people as an indicator of indoor air quality. These uses have included both appropriate and inappropriate interpretations of indoor CO2 concentrations. Appropriate uses include estimating expected levels of occupant comfort in terms of human body odor, studying occupancy patterns, investigating the levels of contaminants that are related to occupant activity, and screening for the sufficiency of ventilation rates relative to occupancy. Inappropriate uses include the application of simple relationships to determine outdoor air ventilation rates per person from indoor CO2 concentrations without verifying the assumptions upon which these relationships are based, and the interpretation of indoor CO2 concentrations as a comprehensive indicator of indoor air quality.
Outdoor air ventilation rates affect contaminant levels in buildings and building occupants' perception of the acceptability of the indoor environment. Minimum rates of outdoor air ventilation are specified in building codes and indoor air quality standards, for example, ASHRAE Standard 62. The compliance of outdoor air ventilation rates with relevant codes and standards are often assessed as part of indoor air quality investigations in buildings. The outdoor air ventilation rate of a building depends on the size and distribution of air leakage sites, pressure differences induced by wind and temperature, mechanical system operation, and occupant behavior. Given all of this information, ventilation rates are predictable; however, many of these parameters are difficult to determine in practice. Therefore, measurement is required to determine outdoor air change rates reliably.
The measurement of CO2 concentrations has been promoted as a means of determining outdoor air ventilation rates per person. This approach, referred to in this guide as equilibrium analysis, is based on a steady-state, single-zone mass balance of CO2 in the building and is sometimes presented with little or no discussion of its limitations and the assumptions on which it is based. As a result, in some cases, the technique has been misused and indoor CO2 concentration measurements have been misinterpreted.
When the assumptions upon which equilibrium analysis is based are valid, the technique can yield reliable measurements of outdoor air ventilation rates. In addition, indoor CO2 concentrations can be used to determine other aspects of building ventilation when used properly. 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.
Continuous monitoring of indoor and outdoor CO2 concentrations can be used to study some aspects of ventilation system performance, the quality of outdoor air, and building occupancy patterns.
Scope
1.1 This guide describes how measured values of indoor carbon dioxide (CO2) concentrations can be used in evaluations of indoor air quality and building ventilation.
1.2 This guide describes the determination of CO2 generation rates from people as a function of body size and level of physical activity.
1.3 This guide describes the experimentally-determined relationship between CO2 concentrations and the acceptability of a space in terms of human body odor.
1.4 This guide describes the following uses of indoor CO2 concentrations to evaluate building ventilation–mass balance analysis to determine the percent outdoor air intake at an air handler, the tracer gas decay technique to estimate whole building air change rates, and the constant injection tracer gas technique at equilibrium to estimate whole building air change rates.
1.5 This guide discusses the use of continuous monitoring of indoor and outdoor CO2 concentrations as a means of evaluating building ventilation and indoor air quality.
1.6 This guide discusses some concentration measurement issues, but it does not include or recommend a method for measuring CO2 concentrations.
1.7 This guide does not address the use of indoor CO2 to control outdoor air intake rates.
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 and health practices and determine the applicability of regulatory limitations prior to use.
