Two working group sessions dedicated to carbon dioxide. The discussions centered around establishing appropriate indoor air quality standards for carbon dioxide levels, considering various perspectives from international regulations, scientific research, and member feedback. This article summarizes the key points of these deliberations and the preliminary direction emerging from the discussions.

Initial Proposals and Rationale

The initial proposal presented to the working group suggested different carbon dioxide ceiling limits for various tiers within the GO AQS framework.

Ultimate Tier: Proposed at 800 parts per million (ppm), aligning with the suggestions of Professor Morawska and her team. This level was largely agreed upon during an earlier voting period, with the primary rationale being to protect occupants from airborne diseases.

Starter Tier: Proposed at 1000 ppm, representing a more relaxed standard for initial assessment of indoor air quality. This tier focuses primarily on PM2.5 and carbon dioxide levels.

These proposals were based on a review of global regulations and guidelines, where 800 and 1000 ppm appeared as common ceiling limits. Green building certificates like RESET also have strict limits, sometimes even below these values, while others, like Fitwel, have seemingly less stringent limits for specific aspects like “viral response”.

Member Feedback and Concerns

Several members voiced their opinions and concerns regarding these proposed limits.

Advocacy for Stricter Limits: A strong point raised by a member, advocated for recognizing indoor air quality as an accessibility issue. This perspective calls for a universally accessible limit, suggesting that all settings should strive for the lowest practicable risk. The feedback specifically suggested setting the starter limit at 800 ppm and the ultimate level at an even lower, “lowest practicable risk” threshold. This viewpoint emphasizes the need to protect vulnerable populations, not just in healthcare settings, but in all public indoor spaces.

International Perspectives: An other member echoed the desire for a global limit to strive towards, even while acknowledging the difficulty in determining an exact standpoint. Belgium currently follows limits of 900 ppm and 1200 ppm. The Spanish approach, which calculates indoor CO2 limits by adding a specific value to the base CO2 level based on the building’s class (IDA 1, 2, 3), was also discussed.

CO2 as an Indicator: Several members highlighted the importance of understanding CO2’s role as a guidance for indoor air quality, noting that while it’s easy to measure, it’s most conclusive for bio-pollutants. Concerns were raised that focusing solely on CO2 might overlook other significant pollutants like VOCs and particulate matter. One participant shared experiences where low CO2 levels coexisted with high levels of other pollutants, leading to occupant discomfort.

Cognitive Function Debate: The ongoing debate within the scientific community regarding whether carbon dioxide levels at typical indoor concentrations affect cognitive function was addressed. The prevailing view among the working group and as per ASHRAE’s recent position document, is to consider CO2 primarily as a tool for assessing disease risk rather than a direct cause of cognitive decline at levels below 2000-3000 ppm. However, it was acknowledged that very high CO2 levels and related factors like tiredness could indirectly impact cognitive functions. A recent study on masks affecting cognitive function due to trapped CO2 was also mentioned.

Ventilation and Energy Implications: The relationship between stricter CO2 limits and the need for increased ventilation was a significant point of discussion. While lower CO2 levels are generally considered better, achieving them through mechanical ventilation can be energy-intensive. Conversely, relying solely on natural ventilation might introduce outdoor pollutants. The importance of considering occupant density and activity levels when determining ventilation requirements was emphasized.

Achievability of Strict Limits: Concerns were raised about the practicality and cost-effectiveness of achieving very low CO2 limits like 600 ppm in various building types. The potential for industry pushback against overly strict standards was also mentioned.

Preliminary Conclusions and Way Forward

While a definitive conclusion on the exact CO2 limits was not reached during these sessions, several key points and a potential direction emerged.

There is a general consensus that the proposed limits of 800 ppm for the ultimate tier and 1000 ppm for the starter tier serve as a reasonable starting point, particularly with the primary goal of mitigating airborne disease risks.

Strong consideration is being given to the feedback suggesting even stricter limits for the ultimate tier and potentially adopting the 800 ppm level for the starter tier as well. This reflects a desire for more universally accessible and health-protective standards.

The working group acknowledges the limitations of using CO2 as the sole indicator of indoor air quality and recognizes the need to consider other pollutants and ventilation rates.

The primary justification for the proposed CO2 limits will focus on their role in assessing disease risk, aligning with the current scientific understanding and the position of organizations like ASHRAE.

Further discussion and consideration will likely involve balancing the desire for stricter air quality standards with the practicalities of implementation, energy consumption, and potential unintended consequences related to ventilation.

The working group will now work towards documenting these proposed numbers and the rationale behind them, making this information public for further debate and feedback. The discussions highlighted the complexity of setting indoor air quality standards and the need to consider multiple factors beyond just carbon dioxide levels. The ongoing dialogue reflects a commitment to developing informed and effective guidelines for healthier indoor environments.

• Recording: Carbon Dioxide WG 1st Session
• Recording: Carbon Dioxide WG 2nd Session