The Significance of Carbon Dioxide (CO2) in Indoor Air Quality (IAQ)

Indoor carbon dioxide (CO2) has long been a topic of discussion in the context of ventilation and indoor air quality (IAQ). Over the centuries, these discussions have evolved to encompass various aspects, including using CO2 as an IAQ metric, estimating ventilation rates through CO2 as a tracer gas, adjusting outdoor air ventilation based on CO2 concentrations, and understanding how CO2 impacts building occupants.

ASHRAE’s Position on Indoor CO2:

The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) has issued a Position Document concerning indoor carbon dioxide in non-industrial indoor environments meant for human occupancy, such as homes, offices, schools, and transportation spaces. ASHRAE takes the following positions:

1. CO2 as an IAQ Metric:

Indoor CO2 concentrations alone do not provide a comprehensive indication of IAQ.

2. Health and Well-being:

The direct impacts of CO2 on health, well-being, learning outcomes, and work performance at typical indoor concentrations lack consistency.

3. Ventilation Standards:

Current indoor CO2 levels do not warrant alterations to ventilation and IAQ standards, regulations, or guidelines.

Using CO2 for Ventilation Evaluation:

Distinguishing between indoor and outdoor CO2 concentrations can be a useful method for assessing ventilation rates and air distribution. This can be achieved through established tracer gas measurement techniques, but accuracy relies on the validity of assumptions and input values.

Critical Factors for Accurate Measurement:

To draw meaningful conclusions from indoor CO2 concentrations, sensor accuracy, proper sensor placement, and regular calibration are crucial considerations.

Air-Cleaning Technologies:

Air-cleaning technologies that exclusively target CO2 removal may not necessarily enhance overall IAQ and could potentially interfere with systems utilizing CO2 for ventilation control or IAQ monitoring.

Research Areas for Better Understanding:
ASHRAE identifies a range of research topics aimed at gaining a deeper understanding of CO2’s role in IAQ.


Indoor CO2 plays a multifaceted role in discussions surrounding IAQ. While it has been considered an IAQ metric and a basis for ventilation evaluation, ASHRAE’s positions suggest that its direct impacts on health and the need for changes in standards are not clear-cut. Further research is needed to comprehensively comprehend the role of CO2 in overall IAQ.

Source: ASHRAE Position Document on Indoor Carbon Dioxide (2022)


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