The Hidden Dangers of Poor Ventilation: Understanding CO2 and Its Impact on COVID-19 Infection Risk
(COVID-19 Series, Part 2 of 7)
Table of Contents
Monitoring Indoor Carbon Dioxide (CO2) Levels: A Strategy for Reducing Risk
Welcome to Part 2 of our 7-part COVID-19 series where we offer solutions to help you avoid diseases that spread through the air -including but not limited to COVID-19, the flu, RSV, and tuberculosis. In Part 1, we discussed the airborne nature of the SARS-CoV-2 virus, including its ability to spread over long distances, build up indoors, and even travel between rooms and floors through structural defects and HVAC systems. We highlighted the urgency of technological advancements in ventilation and filtration to reduce the build-up and spread of airborne diseases in indoor environments.
In this Part 2, we will focus on carbon dioxide (CO2), specifically understanding its relationships to both ventilation and viral infectivity, as well as how CO2 monitoring can be used to assess infection risks, and what actions we can take when those CO2 levels are too high. Prior to moving forward with CO2, let’s briefly describe ventilation.
How does ventilation work?
Ventilation refers to the process of moving air into and out of a building or room, and distributing it for healthy breathing. It involves using natural processes (such as opening windows and doors) to create airflow, and/or mechanical processes (such as using fans or mechanical ventilation systems) to move old air out from a space and then add back fresh air from the outside (or another uncontaminated space). It is also referred to as dilution, since contaminants are diluted with this air movement.
One way to visualize air ventilation is to imagine a goldfish swimming in a bowl of water. Over time the water will get dirtied with waste, algae, and bacteria. You could scoop a cup of dirty water out and add a cup of fresh water back in. This would dilute the pollutants in the water. Air ventilation works in a similar way, changing out dirty air with cleaner air. Of course, if using outdoor air to replace dirty air, it must be clean from pollutants too. During events like wildfire smoke, outdoor air should not be used. See our blog: Indoor Air Quality, COVID-19, and Wildfire Smoke to learn more about what to do during wildfire smoke events. Ideally, there will be a continual inflow of fresh air and outflow of stale or contaminated air in an amount that will reduce infectious aerosols (like COVID-19), as well as other toxins and contaminants in the air.
The Origins of Carbon Dioxide Indoors
Carbon dioxide (CO2) is a colorless, odorless gas that is naturally present in the Earth’s atmosphere. However, its concentration can increase significantly indoors, particularly in poorly ventilated spaces. One of the primary sources of CO2 indoors is human respiration. Every time we breathe, we inhale oxygen and exhale carbon dioxide. In enclosed environments, especially those with many people, CO2 levels can rise quickly if there is insufficient ventilation to disperse the gas.
Additionally, CO2 can also originate from various household appliances and activities [1]. Gas stoves, heaters, fireplaces, and even burning candles contribute to indoor CO2 levels. Combustion processes, whether from cooking or heating, release CO2 into the air, which can accumulate over time, particularly in areas with poor air exchange. Understanding these sources is crucial in managing and improving IAQ, as high CO2 levels have many health implications.
How Carbon Dioxide Levels Indicate Ventilation Quality
First of all, carbon dioxide (CO2) levels in indoor environments can serve as a proxy for ventilation quality. In well-ventilated spaces, CO2 levels are typically low, reflecting a high rate of fresh air exchange. Conversely, elevated CO2 levels indicate poor ventilation and a higher potential for the buildup of contaminants, including airborne viruses. In their 2023 Ventilation in Buildings update, the Centers for Disease Control and Prevention (CDC) recommends maintaining under 800 parts per million (ppm) as a target for CO2 indoors [2].
How Do We Monitor CO2 in Indoor Environments and What Do the Numbers Mean?
CO2 monitors that measure carbon dioxide, the gas we exhale (not to be confused with CO monitors that measure carbon monoxide from things like gas generators) have become an increasingly popular tool to use in classrooms, offices, homes, and on public transportation for measuring real-time CO2 concentration. They are small, often portable, and relatively inexpensive non-dispersive infrared (NDIR) CO2 monitors. Two popular brands are Aranet and Inkbird. You can learn more about different types of CO2 monitors here [3].
CO2 monitors measure the CO2 in the air in parts per million (ppm). CO2 is more likely to build up to higher levels in places that do not have fresh air coming in and stale air going out. It is a problem on its own when it is high, because higher CO2 is associated with health impacts. It is also used as a proxy for the associated risks of buildup of particulates – including viral aerosols like COVID-19.
Currently, the background level of CO2 in the typical ambient outdoor air is approximately 400 ppm. Its indoor concentration mainly depends on the ventilation rate and human population density. In poorly ventilated spaces, the CO2 emitted by breathing accumulates over time, often raising levels much higher than the recommended 800-1000 ppm. The problem is magnified in modern times due to more buildings using air conditioning and being built to reduce leakage of air for energy efficiency. For example, the CO2 concentration in unventilated bedrooms can be as high as 2500 ppm. In classrooms, the CO2 concentration can exceed 4000 ppm without adding ventilation improvements. Recent research has shown that even moderately elevated levels of CO2 can impact human health and cognitive function, especially in children, emphasizing the need to maintain low CO2 levels indoors [4].
Although CO2 cannot predict whether an infectious person is in the room, it can serve as a proxy for infection risk in a room. Because air filtration devices like HEPA purifiers do not filter out CO2, a place may have well-filtered air in terms of reducing infection risks, but still measure higher than the recommended 800 ppm of CO2. Therefore, if HEPA purifiers or other filtration devices are in use, aim for under 1000 ppm as a marker for infection risks. Higher than 1000 ppm of CO2 can cause CO2-related health risks, and actions should be taken to ventilate the CO2 out and introduce fresh air.
Health Implications of Poor Ventilation
Poor indoor air quality (IAQ) has far-reaching health impacts, from exacerbating respiratory conditions to increasing susceptibility to airborne viruses. Elevated CO2 levels (over 1000 ppm) can lead to symptoms such as headaches, dizziness, and fatigue. Moreover, cognitive functions are particularly sensitive to high CO2 levels, affecting daily activities and productivity [5].
Opening windows, even just 6 inches, or for a few minutes each hour can make a significant difference, often adding between 1-4 air changes per hour (ACH) [6] [7]. This amount can be helpful for reducing cognitive effects and be part of a process of getting the ACH needed to reduce COVID-19 infection risks, as we will talk more about later.
The Impact of High Carbon Dioxide Levels on Cognition
One immediate health risk of high indoor CO2 levels is their significant impact on cognitive function. Studies have shown that elevated CO2 levels can impair decision-making, reduce concentration, and slow reaction times. For instance, a study involving chess players revealed that higher CO2 concentrations led to delayed reaction times and adversely affected their strategic decision-making abilities [8]. This phenomenon occurs because high CO2 levels can reduce the amount of oxygen available to the brain, impacting its function and efficiency [5].
A 2016 study [9] that looked at attention levels (measured as “Power of Attention”) on children in classrooms found that increasing levels of CO2 (from a mean of 690 ppm to a mean of 2909 ppm) led to a decrease in Power of Attention of approximately 5%. Researchers concluded:
In a classroom where CO2 levels are high, students are likely to be less attentive and to concentrate less well on what the teacher is saying, which over time may possibly lead to detrimental effects on learning and educational attainment.
A 2015 study [10] by Joseph Allen, et al. that examined cognitive effects of workers at different levels of CO2 found compared to their work on green days (CO2 below 800 ppm), workers received a 15% lower cognitive ability score at 950 ppm, and a further 50% decline at 1400 ppm.
Furthermore, there is compelling evidence from both cross-sectional and intervention studies of an association of increased student performance with increased ventilation rates. There is also evidence that increased ventilation rates reduce respiratory symptoms and student absences [11].
How Does Carbon Dioxide (CO2 Levels) Increase COVID-19 Infection Risks?
Besides finding that CO2 levels are used as an indicator of poor ventilation, which can result in higher build-up of infectious aerosols in the environment, recent research has also shown that higher CO2 levels can increase the amount of time the SARS-CoV-2 virus remains infectious in the air.
For example, a 2024 Nature study showed that increasing the CO2 concentration to just 800 ppm, a level widely identified as being a marker of good ventilation, significantly extended the life spans of multiple SARS-CoV-2 variants. At higher concentrations, like those regularly found in rooms with higher occupancy and lower ventilation, the researcher’s team found that 10 times as much virus remained infectious after 40 minutes when compared to rooms with under 800 ppm CO2 [12].
When CO2 levels are high, the air can sustain a higher concentration of infectious airborne viral particles, making it easier for these particles to be inhaled by individuals, thus increasing the chance of transmission, and further exacerbating the spread of COVID-19.
A 2023 study [13] takes this information a step further by demonstrating the relationship between indoor CO2 levels and the probability of developing COVID-19 in a real-world setting. As you can see in the chart below, infection probability rises sharply with increasing CO2 levels, starting from about 30% at 400 ppm to nearly 100% at 2000 ppm in the absence of mitigations such as masking. This visual representation underscores the critical need to maintain low CO2 levels indoors, as higher levels significantly increase the risk of viral transmission.
The chart above illustrates how high levels of indoor carbon dioxide (CO2) can significantly elevate the infection probability among individuals in shared spaces, especially when they are not masked, but also considerably when wearing surgical masks as opposed to N95s. This revelation highlights an urgent need to improve ventilation in our indoor environments to protect public health.
Practical Measures to enhance ventilation
First of all, carbon dioxide (CO2) levels in indoor environments can serve as a proxy for ventilation quality. In well-ventilated spaces, CO2 levels are typically low, reflecting a high rate of fresh air exchange. Conversely, elevated CO2 levels indicate poor ventilation and a higher potential for the buildup of contaminants, including airborne viruses. Recall that earlier we mentioned that the CDC recommends maintaining under 800 parts per million (ppm) as a target for CO2 indoors. Here are some things you can do to keep your CO2 levels within this range [2].
Practical Steps to Improve Ventilation:
- Increase Natural Ventilation: Open windows and doors to allow fresh air to circulate. This simple step can significantly reduce indoor CO2 levels and improve air quality.
- Use Exhaust Fans: In areas where natural ventilation is limited, exhaust fans can help expel stale air and bring in fresh air.
- Monitor CO2 Levels: Use CO2 monitors to track indoor air quality. Aim to keep CO2 levels below the 800-1000 ppm range to ensure a safe and healthy environment.
- Implement Air Filtration Systems: While this blog focuses on ventilation, combining it with air filtration is often necessary to achieve the air changes per hour needed to reduce the risk of airborne virus transmission. Recall that because air filtration does not filter out CO2, its air quality improvements, including those for reducing viruses in the air, cannot be seen on CO2 monitor measurements. Aim for under 1000 ppm instead of under 800 ppm for infection risks when air filtration is used.
Conclusion: The Significance of Ventilation in COVID-19 Mitigation
Addressing IAQ is essential in the fight against airborne diseases like COVID-19. Improved ventilation, as indicated by lower CO2 levels, plays a vital role in reducing infection risks and protecting public health. By understanding and managing indoor CO2 levels, we can create healthier and safer environments for everyone. It is imperative that we adopt practical measures to enhance ventilation in our homes, schools, and workplaces. This includes not only the simple act of opening windows and doors, but also utilizing technology to monitor and maintain optimal air quality. By prioritizing these actions, we not only mitigate the spread of COVID-19, but also pave the way for healthier indoor environments in the future. The health of our communities depends on the air we breathe, and it is our collective responsibility to ensure it is clean and safe.
The adverse effects of high CO2 levels on cognitive function further highlight the importance of this issue. From impaired decision-making to reduced reaction times, the implications for daily activities and overall well-being are significant. Therefore, enhancing ventilation and monitoring CO2 levels should be a priority for everyone, ensuring we maintain environments that support both our physical health and cognitive performance.
Building on the exploration of carbon dioxide (CO2) levels and ventilation, our next installment (Part 3: Ventilation and Filtration: Key to Reducing Viral Density) dives into the practical prevention measures that can be taken using ventilation and filtration to reduce the density of virus-containing aerosols in indoor environments.
“When you feel like life is out of focus, always return to the basics of life. Breathing. No breath, no life.”
[1] Sources of greenhouse gas emissions | US EPA. (2024b, July 8). US EPA. https://www.epa.gov/ghgemissions/sources-greenhouse-gas-emissions
[2] CDC. “Ventilation in Buildings.” Centers for Disease Control and Prevention. https://www.cdc.gov/coronavirus/2019-ncov/community/ventilation.html
[3] Brooke, E. (2024, April 3). Carbon dioxide monitors – What You need to know + 8 best CO2 Monitors. Breathe Safe Air. https://breathesafeair.com/carbon-dioxide-monitors/
[4] “Impact of Indoor CO2 on Human Health.” ScienceDirect. https://www.sciencedirect.com/science/article/pii/S036013232300358X#:~:text=However%2C%20recent%20studies%20have%20shown,below%205000%20ppm%20%5B10%5D
[5] Azuma, K., Kagi, N., Yanagi, U., & Osawa, H. (2018). Effects of low-level inhalation exposure to carbon dioxide in indoor environments: A short review on human health and psychomotor performance. Environment International, 121, 51–56. https://doi.org/10.1016/j.envint.2018.08.059
[6] HowardReed, C, L A. Wallace, AND W R. Ott. THE EFFECT OF OPENING WINDOWS ON AIR CHANGE RATES IN TWO HOMES. JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION 52(2):147-159, (2002). https://cfpub.epa.gov/si/si_public_record_report.cfm?dirEntryId=65288&Lab=NERL
[7] Pandey, P., & Dong, A. B. (2023). Analysis of Impacts of Window Opening Behavior on Indoor Air Pollutants in Residential Dorms through Deep Neural Network. E3S Web of Conferences, 396, 01097. https://doi.org/10.1051/e3sconf/202339601097
[8] Künn, S., Palacios, J., & Pestel, N. (2023). Indoor air quality and strategic decision making. Management Science, 69(9), 5354–5377. https://doi.org/10.1287/mnsc.2022.4643
[9] Coley, D. A., Greeves, R., & Saxby, B. K. (2007). The effect of low ventilation rates on the cognitive function of a primary school class. International Journal of Ventilation, 6(2), 107–112. https://doi.org/10.1080/14733315.2007.11683770
[10] Allen, J. G., MacNaughton, P., Satish, U., Santanam, S., Vallarino, J., & Spengler, J. D. (2016). Associations of Cognitive Function Scores with Carbon Dioxide, Ventilation, and Volatile Organic Compound Exposures in Office Workers: A Controlled Exposure Study of Green and Conventional Office Environments. Environmental Health Perspectives, 124(6), 805–812. https://doi.org/10.1289/ehp.1510037
[11] Fisk, W. J. (2017). The ventilation problem in schools: literature review. Indoor Air, 27(6), 1039–1051. https://doi.org/10.1111/ina.12403
[12] Haddrell, A., Oswin, H., Otero-Fernandez, M., Robinson, J. F., Cogan, T., Alexander, R., Mann, J. F. S., Hill, D., Finn, A., Davidson, A. D., & Reid, J. P. (2024). Ambient carbon dioxide concentration correlates with SARS-CoV-2 aerostability and infection risk. Nature Communications, 15(1). https://doi.org/10.1038/s41467-024-47777-5
[13] Iwamura, N., & Tsutsumi, K. (2023). SARS-CoV-2 airborne infection probability estimated by using indoor carbon dioxide. Environmental Science and Pollution Research International, 30(32), 79227–79240. https://doi.org/10.1007/s11356-023-27944-9
Sarah Masih, MD, MBA
Sarah Masih is the Executive Director of Air Support Project and has extensive experience in project management, fundraising, and communications. Her deep commitment to public health and her collaborative approach with community organizations and health experts make her a vital force in the fight for cleaner air and healthier communities.
SHEA O'NEIL
Volunteer
Creator of COVID-conscious advocacy works: writings, art, and social networking. BA degree in psychology, with over 11 years of continued education studies in allergy and autoimmune research, and continued research the past 3 years into airborne irritants and COVID-19. Parent, disabled rights advocate.
S. Riley
S. Riley is a mutlidisciplinary content creator and technology enthusiast with a background in crowdfunding and project management. He holds a BA in English, and is grateful to be able to utilize his unique skill set in service of Air Support Project's humanitarian mission.
Melissa Smallwood
Melissa Smallwood is Science and Technology Policy researcher with a background in neuroscience, psychology, and social science, and the author of The Future of Long COVID: A Threatcasting Approach. They are interested in how disability and healthcare justice intersect with emerging technologies, and are an advocate for COVID mitigation strategies to help end the pandemic.
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CO2 LEVELS CHART
A chart that defines the different ranges of CO2 and impacts on health and infection risks.
CO2 INFECTION RISKS
This graph shows the increased infection risks associated with higher CO2 levels.
CO2 TRACKER
A downloadable chart for you to track your space's CO2 levels throughout the day.
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