The Air We Breathe: The Significance of Carbon Dioxide Monitoring
—written by Chen Chen
The COVID-19 pandemic has been a challenging time for many people, particularly those who are immunocompromised or have pre-existing health conditions. As restrictions on indoor gatherings and mask mandates were lifted, engineer and clean air advocate Cheryl White wanted to ensure that resources for improving air quality were accessible to everyone as they ventured back into public spaces.
Mining foreman R. Thornburg shows a small cage with a sentinel canary used for testing carbon monoxide gas in 1928. Today, although our detectors still chirp, they take the form of electrochemical sensors instead of live birds. Credit: Wikimedia commons
Few thought about indoor air quality before the pandemic. Now, heavy consideration is placed on air filtration and ventilation within buildings. According to Dr. Krystal Pollitt, an Associate Professor of Epidemiology from the Yale Institute of Global Health, we spend about 90% of our time indoors, a statistic that is fairly consistent across all age groups. In addition to the risk of airborne pathogens such as SARS-CoV2, it is now clear that everyday living can generate particles and gasses that endanger people if allowed to accumulate.
Carbon dioxide (CO2) is often used as a proxy for monitoring air quality indoors, specifically ventilation. It can be an indicator of how much of each other’s air people are exchanging. Cheryl began using CO2 monitors as a way to learn about the characteristics of air in different types of venues in an effort to make better decisions about where to spend time indoors. As much as she relied on the monitors, she was also aware that access to monitors and public knowledge on the matter was limited. Cheryl took to Twitter and proposed the idea of a carbon dioxide monitor lending program to the Toronto Public Library. What started as a single tweet in October 2021 has now grown into an initiative reaching citizens across Canada. Alongside Danielle Cane (MSc Infectious Diseases from London School of Hygiene and Tropical Medicine) and Kate Nyhan (MLS, IPI, public health librarian at Yale University), Cheryl co-founded the Community Access to Ventilation Information (CAVI) program. CAVI’s mission? To make quality carbon dioxide monitors more accessible to the general public, while increasing public awareness around indoor air quality.
An example display screen of an Aranet4 CO2 monitor shows the space around the sensor has 3,255 ppm of CO2. As this concentration is well above the recommended 1,000 pm, if it persists there may be concerns about poor ventilation. Credit: Aranet 4
Carbon Dioxide Monitoring
Several types of carbon dioxide monitors are currently available, including electrochemical and metal oxide monitors. Aranet4, a popular model available through the Toronto Public Library lending program, uses a non-dispersive infrared (NDIR) sensor. An infrared lamp directs light along a tube in the sensor as the air moves through it towards a detector. The concentration of CO2 in this air is calculated based on the light absorbed at the specific wavelength or signature of CO2 molecules.
Aranet4 displays CO2 concentration in units of parts per million (ppm). “The Public Health Agency of Canada recommends people have an average CO2 level in their homes at 1,000 ppm,” explains Heating, Ventilation, Air Conditioning (HVAC) engineer Joey Fox. Lower thresholds are mandated in schools and hospitals, while average outdoor air sits at 420 ppm.
While measuring CO2 concentrations can be a useful proxy for air quality, many scientists caution against obsessing over exact ppm values. Instead, users are recommended to evaluate relative CO2 concentrations over time and assess potential solutions for lowering their levels. Carbon dioxide is predominantly produced indoors from human exhalation and other sources such as combustion in gas stoves. “Ventilation is the main way to control CO2 and air quality,” remarks Fox. Users can try opening windows to reduce CO2 levels in a room and evaluate the effectiveness of the action using a CO2 monitor.
In an indoor space, multiple factors affect COVID-19 exposure risk in addition to CO2 concentration. “If everyone in the space is wearing a really good mask, then CO2 level would be exactly the same but the respiratory risk is lower than if no one is wearing a mask,” explains Prof. Jeffrey Siegel. “Same thing if there is very good particle filtration in the space. You will see potentially high level of CO2 but not very high risk because [disease-causing agents] are removed by the filters.”
Ventilation is Key
Fox has reviewed the quality of numerous ventilation units, with 10 years’ experience optimizing ventilation in public schools. Poorly programmed ventilation units, including those with duct dampers that completely close when indoor temperatures become too warm during wintertime, can contribute to the spread of illnesses by preventing adequate ventilation of the space. Conversely, Fox has observed that schools with stable CO2 concentrations over time are likely using a programmable fan-controlled ventilation system where you can set the CO2 level. Whether through a balancer or re-programming, it is crucial to maintain appropriate air flow in the classroom.
Research supports that poor ventilation, measured indirectly through fluctuations and spikes in carbon dioxide levels, can impact cognitive function. “When you reduce ventilation, you see things like lower performance on standardized tests, lower grades, worse cognitive assessments,” warns Jeffrey Siegel, Professor of Civil and Mineral Engineering at the University of Toronto. Additionally, emergency room physician Dr. Kashif Pirzada cautions that poor air quality is also associated with more people presenting with exacerbated lung injuries or lung conditions like chronic obstructive pulmonary disease (COPD) and chronic emphysema.
Improving Air Quality
In conjunction with expanding carbon dioxide monitor accessibility and awareness, Dr. Kashif Pirzada co-developed the RAVEN CleanAir map. Built on top of the RAVEN app, a news and social media aggregator, this CleanAir map allows users to share and view CO2 readings in public venues along with comments about the number of occupants, room volume, level of activity within the space, filtration, and much more. The map has been expanded to over a thousand users in 15 countries, and is steadily growing. Dr. Pirzada hopes the app can reach venue owners to inspire action towards improving air quality and highlight the systemic issues with certain categories of indoor spaces. “One trend we discovered is that transit systems are not very good at controlling CO2,” Dr. Pirzada remarks, “and some of the most egregious on our map are entertainment spaces. We saw one reading that was about 5,000 ppm.”
According to recent Raven CleanAir map users, CO2 concentrations in the Toronto Reference Library remain low for a public indoor space. Credit: RAVEN (screencaptures by Chen Chen)
Looking beyond carbon dioxide, Dr. Pollitt reminds us of other indoor air pollutants, such as plasticizers used in stain-resistant coating on carpets or flame retardants in clothing, which outgas a huge range of chemicals that contribute to a range of health outcomes. With all these chemicals to consider, understanding ventilation, filtration and other air quality optimization measures are important to limit exposure to harmful particles and respiratory viruses.
Ongoing research will continue to identify key air quality challenges and propose innovative solutions. In the meantime, pilot programs like the one started by CAVI provide the public with the tools and knowledge to monitor air quality in spaces they frequent. Through CAVI’s partnerships with public libraries, over 27% of Canadians now have access to CO2 monitors. With continued community support, more institutions can connect Canadians with crucial resources for better health and safety in public spaces.