Application of Indoor Carbon Dioxide During the COVID- 19 Pandemic

In response to the COVID-19 pandemic, many organizations have recommended improved ventilation to reduce the risk of indoor airborne infectious disease transmission. These recommendations include increasing outdoor air rates and filtration efficiencies, as well as verifying that ventilation systems are operating as intended. There have also been many recommendations to monitor indoor CO2 concentrations as indicators of ventilation or infection risk, in some cases with quantitative concentration limits. However, the technical basis for these recommendations to monitor CO2 and, more importantly, the basis for the concentration limits are not always clear. CO2 monitoring and analysis has also been used in many research studies of the risks of airborne disease transmission and the potential effectiveness of mitigation measures. This paper reviews research applications of indoor CO2, as well as recommendations for CO2 monitoring and interpreting measured concentrations issued during the pandemic. As described in the paper, some of the research applications and recommendations employ CO2 as an indicator of the adequacy of outdoor air ventilation rates, essentially an application of well-established tracer gas measurement methods. In other cases, CO2 is used as a proxy for exposure to infectious aerosols. In yet other cases, the motivation for measuring indoor CO2 concentrations and recommended levels is not well explained. This paper reviews the application of indoor CO2 in response to pandemic and raises several questions regarding their technical basis and the potential for improvement.