Limitations of CO2 measures as an indicator of indoor air quality

• Carbon dioxide (CO₂) is monitored to assess air quality and adequate ventilation indoors
  • Human activity indoors increase levels of CO₂
  • CO₂ measures are also used as an indicator for hard-to-detect volatile organic compounds (VOC) and microorganisms
  • Ventilation settings are then calculated accordingly to prevent CO₂ buildup, and also reduce exposures to potentially harmful compounds and pathogens

• Indoors CO₂ acceptable standards has been arbitrarily established at around 1000 ppm
  • Nonetheless, recent reviews have failed to demonstrate a clear link between increasing CO₂ concentration and lower reported quality of air and associated symptoms¹
  • Furthermore, a study has shown that CO₂ linked to human activity had effects on perceived air quality and cognitive impairment, but that pure CO2 at the same concentration did not have these effects, suggesting that something else related to human activity (breathing, talking) was responsible for the symptoms reported²

• Thus, CO₂ measures might not be an adequate indicator of airborne biological contaminants, such as bacteria and viruses:
  • Historically, many studies have demonstrated a link between adequate ventilation (as per CO₂ levels) and lower transmission of airborne diseases.
  • Yet, multiple factors can affect CO₂ emissions and have little to no effect on increasing airborne microbial count.
    • Animals and other living beings
    • Combustion devices
  • Also, some factors can increase or decrease the airborne charge of biological contaminants, but have no effect on CO₂ emissions
    • Superspreaders; People that emit high charges of airborne pathogens, but don’t produce more CO₂
    • Singing and talking has been linked to an increase in emission of viral particles
    • Wearing a mask reduces the emission of airborne bio-contaminants, but has no effects on CO₂ production
    • Air disinfection and filtration have no effect on CO₂ levels but reduce airborne particle charge

In summary, even though CO₂ measures and monitoring can be installed at low cost and used to evaluate ventilation, it might not be an adequate measure of the risk of exposure to airborne pathogens (such as SARS-CoV-2). It does, however, helps to educate the public about the importance of improving ventilation and indoor air quality (IAQ).

¹ https://eta-publications.lbl.gov/sites/default/files/ashrae_journal_-_september_2019_76_-_77.pdf

² https://onlinelibrary.wiley.com/doi/abs/10.1111/ina.12284