Carbon dioxide (CO2) concentration is a critical indicator of indoor air quality (IAQ) and plays a significant role in building performance. While CO2 itself may not be directly harmful at typical indoor levels, its concentration serves as a proxy for ventilation effectiveness and the potential buildup of other indoor air pollutants, including pathogens, that can negatively impact occupant health and cognitive function. This article examines the relationship between CO2 levels and building performance, drawing upon research findings and expert insights to recommend CO2 levels that promote a healthy and productive indoor environment.

Understanding the Role of CO2 in Indoor Environments

CO2 is a natural component of the air we breathe, but its levels can increase significantly in indoor spaces due to human respiration and other combustion processes (candles, cooking, combustion heating, etc). When ventilation is inadequate, CO2 from these sources can accumulate, leading to elevated concentrations.

Many sources recognize the significance of CO2 as an indicator of ventilation effectiveness. Elevated CO2 levels often signal inadequate ventilation, suggesting a potential buildup of other indoor air contaminants that could negatively impact occupant health and cognitive function. This connection is established by the presence of both CO2 and pathogens in exhaled breath, which ventilation systems aim to remove from indoor spaces.

CO2 and Cognitive Function

Research has shown a correlation between elevated CO2 levels and impaired cognitive function. Studies have reported a decline in decision-making performance, particularly in complex tasks, starting at CO2 concentrations of around 1000 ppm. For instance, one study revealed a substantial 165% decrease in performance on an attention test when CO2 levels rose to between 1000 and 2000 ppm due to reduced ventilation. While accuracy remained unaffected in this specific case, the marked reduction in speed highlights the potential advantages of improved ventilation to mitigate the effects of CO2 and other pollutants.

High CO2 levels can lead to a reduction in attention and concentration in schoolchildren. A study showed that increasing CO2 levels from 690 ppm to 2909 ppm caused a 5% decrease in the “Power of Attention,” a measure of concentration intensity.

On the other hand, another study looked at levels up to 3000 ppm, and they concluded that they did not significantly impact perceived air quality, acute health symptoms, or cognitive performance. This finding challenges previous studies that suggested a direct negative effect of CO2 on cognitive function. The authors suggest that previous findings may be attributed to methodological differences, particularly the complexity of the cognitive tests employed.

Secondary analysis from another study suggests that air quality does have an effect on processing accuracy. Air quality (low CO2 levels) could be more important to precision than to processing speed.

CO2 and Pathogens Indoors

Research using the CELEBS technique demonstrates that elevated CO2 levels (e.g., 800 ppm) significantly enhance the aerostability of SARS-CoV-2, an effect even more pronounced than that of relative humidity. This increased aerostability means that a greater fraction of viral infectious particles remain viable in the aerosol phase at elevated CO2 concentrations compared to well-ventilated environments. It is also suggested that the seasonality of respiratory viral infections could be linked to fluctuations in indoor CO2 concentrations, in addition to the traditionally considered factor of relative humidity.

Factors Influencing Optimal Building Performance

It’s important to understand that CO2 is only one factor contributing to a healthy and productive indoor environment. Several other factors play a role, including:

Ventilation rate: Adequate ventilation is crucial to remove CO2 and other indoor air pollutants. Ventilation rate is often measured in liters per second per person (L/s/p) or air changes per hour (ACH).

Temperature: Optimal indoor temperatures for cognitive performance typically fall within a relatively narrow range.

Other pollutants: Volatile organic compounds (VOCs), particulate matter (PM2.5), and other indoor pollutants can also impact health and cognitive function.

Individual sensitivity: People may have different sensitivities to CO2 and other indoor environmental factors.

Recommended CO2 Levels

While the sources reviewed for this article don’t definitively state a universally applicable recommended CO2 level for optimal building performance, they provide a foundation for suggesting a reasonable target. While a CO2 level below 800 ppm appears to be a prudent goal for supporting cognitive function and overall well-being in buildings, levels up to 1000 ppm may be acceptable in buildings where energy efficiency and conservation are prioritized.

It is important to note that this recommendation is based on a limited number of studies and that further research is needed to establish definitive guidelines.

Additional Considerations

In addition to the recommended CO2 level, several other factors should be considered when aiming for optimal building performance:

ASHRAE Standards: Industry standards, such as ASHRAE Standard 62.1, provide guidelines for ventilation rates and IAQ in buildings. These standards should be consulted to ensure compliance and best practices.

Building Type: Different building types may have different requirements. Schools, offices, and residential buildings have unique occupancy patterns and activity levels that influence ventilation needs.

Occupancy Levels: Higher occupancy levels require increased ventilation to maintain acceptable CO2 concentrations.

Monitoring and Control: Regular CO2 monitoring and the use of demand-controlled ventilation systems can help optimize ventilation rates and energy efficiency.

Conclusion

While CO2 itself may not be directly harmful at typical indoor concentrations, it serves as a valuable indicator of ventilation adequacy and the presence of other potentially harmful bioeffluents. Maintaining CO2 levels below 800 ppm in buildings is a good starting point for promoting good IAQ. However, it’s crucial to consider building-specific factors, applicable standards, and individual needs when setting and maintaining appropriate indoor environmental conditions. Achieving optimal building performance requires a holistic approach that considers multiple factors and prioritizes the health and well-being of occupants.

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