Indoor carbon dioxide (CO2) has been a central element in discussions about building ventilation and indoor air quality (IAQ) for centuries. From early scientific investigations to modern building management practices, our understanding of CO2’s role has evolved significantly. This article explores the complex relationship between CO2 in indoor environments, ventilation strategies, and occupant health based on ASHRAE’s latest position document on this topic.
Throughout this exploration, it becomes evident that monitoring CO2 levels represents just one piece of the indoor air quality puzzle. A comprehensive approach to IAQ management requires the monitoring of multiple parameters beyond CO2. By integrating different monitoring systems, such as Remotair, it is possibile to track several pollutants simultaneously, enabling the maintainance of optimal indoor environment status through data-driven control strategies.
The historical context of CO2 monitoring
The relationship between CO2 and indoor environments dates back to the 17th century. At that time, researchers like Mayow proposed that “igneo-aerial particles” produced by candles caused harm to animals. By the 18th century, Lavoisier attributed these effects specifically to CO2. During this period, CO2 was considered both a cause of physiological effects and an indicator of air staleness.
In the 19th century, Max Josef von Pettenkofer shifted this understanding, suggesting that CO2 itself wasn’t causing discomfort, but rather served as a surrogate marker for “vitiated air” containing deleterious substances from human skin and lungs. Pettenkofer proposed 1000 ppmv as a threshold marker for inadequate indoor ventilation. This value continues to influence discussions today, though often without understanding its original context.
CO2 as a ventilation indicator: capabilities and limitations
Modern building science recognizes that indoor CO2 concentrations can serve as useful indicators of ventilation rates per person, but with important cautions. As ASHRAE’s position document clarifies, CO2 levels are not comprehensive indicators of overall IAQ. Here’s why:
- Source-specific limitations: many significant indoor pollutants originate from building materials, furnishings, cleaning products, and outdoor sources – all of which are unrelated to occupancy and therefore not correlated with CO2 levels;
- Differences in removal mechanisms: while CO2 is primarily removed through ventilation, other pollutants may be removed through additional mechanisms such as deposition, filtration, or photocatalytic decomposition;
- Spatial variations: CO2 concentrations can vary significantly within a space based on occupancy patterns and air distribution characteristics
The frequently cited 1000 ppmv threshold deserves particular attention. This value isn’t a health-based limit but corresponds to an outdoor air ventilation rate of about 8 L/s (16 cfm) per person. This rate has historically been associated with to the perception of human body odour, rather than the direct health impacts of CO2 itself. Applying this single threshold to all building types misunderstands ventilation requirements, which vary according to space type, occupant characteristics, and activity levels.
Health and cognitive effects of CO2 in indoor environments
Recent research has explored whether CO2 itself, at concentrations typically found indoors (600-5000 ppmv), might directly affect human health and cognitive function. The results paint a complex picture. Indeed, several studies have reported associations between elevated CO2 (around 1000 ppmv) and reduced cognitive performance, with some showing concentration-dependent effects. However, other studies have found no cognitive impacts.
Some research with mice has identified inflammatory changes at 2000-4000 ppmv, including neutrophil activation in blood and fluid leakage from blood vessels into brain tissue. Similar inflammatory responses have been observed in limited human studies, potentially supporting the biological plausibility of cognitive effects.
However, ASHRAE believes that the current evidence for direct effects of CO2 on health, well-being, learning outcomes, and work performance at typical indoor concentrations remains inconsistent and insufficient to justify changes in ventilation standards or guidelines. These inconsistencies indicate the need for further investigation into potential mechanisms and confounding factors.
Practical applications of CO2 monitoring
Despite limitations as a general IAQ metric, CO2 monitoring serves several valuable purposes in building management:
- Demand-Controlled Ventilation (DCV): CO2-based DCV adjusts outdoor air intake rates based on actual occupancy rather than design occupancy, potentially saving substantial energy while maintaining adequate ventilation. Some energy efficiency standards are now requiring this approach, including ANSI/ASHRAE/IES Standard 90.1;
- Ventilation Rate Measurement: CO2 can function effectively as a tracer gas for estimating ventilation rates when applied properly. There are two primary methods: decay method and steady-state method: the decay method and the steady-state method. Both approaches require careful attention to underlying assumptions, including uniform mixing, constant outdoor concentration, and known CO2 generation rates by occupants. Reporting the uncertainty of these measurements is essential, particularly when spaces don’t conform to single-zone assumptions;
- Infection Risk Assessment: during the COVID-19 pandemic, there was a surge of interest in using CO2 monitoring to assess the risk of airborne disease transmission. Several organizations issued guidance linking CO2 concentrations to infection risk. Though, ASHRAE does not recommend specific concentration thresholds for this purpose.
The CO2 removal paradox
With growing interest in indoor CO2 levels, technologies specifically targeting CO2 removal have entered the market. However, these present a paradox: removing CO2 without addressing other pollutants may create a false sense of IAQ improvement while interfering with CO2-based ventilation control systems.
For comprehensive IAQ management, building professionals should recognize that CO2 removal alone doesn’t address other important contaminants and different air-cleaning technologies target different pollutants. Moreover, CO2-based DCV systems assume measured CO2 reflects occupancy, which is distorted by CO2 scrubbers.
CO2 in indoor environments: conclusion
Indoor CO2 remains an important consideration in building ventilation and IAQ management, although its importance is often misunderstood. While CO2 monitoring is not a comprehensive IAQ indicator, it does provide valuable insights when used properly and with an understanding of its limitations.
Building professionals should approach CO2 monitoring with nuanced understanding – recognizing its utility for ventilation assessment and control while acknowledging that maintaining low CO2 levels doesn’t guarantee good overall IAQ. As research continues and outdoor concentrations rise, our approach to indoor CO2 will continue to evolve.
Modern monitoring systems such as Remotair represent an important advancement in this field, enabling more sophisticated approaches to CO2 management through continuous, integrated monitoring capabilities. These technologies help to bridge the gap between the theoretical understanding of the role of CO2 in indoor environments and the practical implementation of effective ventilation strategies.
By applying current best practices in CO2 monitoring and keeping informed about emerging research, building professionals can leverage this parameter effectively within a comprehensive strategy for healthy, efficient indoor environments.
This blog post summarizes key points from ASHRAE’s Position Document on Indoor Carbon Dioxide (February 2025). For complete information, please consult the original document. To keep up to date with the latest IAQ and HVAC publications, visit our news section.
