TA-23-C041 – Pitfalls of Using CO2 Measurements as the Sole Indicator of IAQ and Airborne Transmission Risk PDF

Measurements of indoor CO₂ concentration provide valuable information about the indoor environment. Steady-state readings give a high-quality estimate of the ratio between generation (from occupants) and removal (via ventilation), while timeseries transient readings can be used to estimate ventilation rates. Both values are key to monitoring whether the HVAC system is maintaining adequate IAQ, and they can be used as inputs to models that estimate the risk of airborne transmission of infectious aerosols (e.g., containing SARS-CoV-2, which is responsible for the COVID-19 pandemic). The increasing availability of CO₂ sensors has led to a push to require CO₂ monitoring in schools and other public places. Such monitoring would provide valuable data to assess IAQ. Unfortunately, this push is also accompanied by a desire to define fixed thresholds of CO₂ concentration that separate “good” and “bad” IAQ as well as “safe” and “unsafe” conditions with respect to airborne transmission. Such approaches are misguided, as CO₂ concentration by itself does not provide sufficient context to fully evaluate IAQ or transmission risk. Our purpose in this paper is to explain why CO₂ concentration provides a partial but fundamentally incomplete picture of the indoor environment and to discuss the other key factors needed to perform a thorough assessment. For IAQ, we show how CO₂ is a surrogate only for human-sourced pollutants, which means a space with low CO₂ concentration could still have an unacceptably high concentration of building-sourced pollutants. For airborne transmission risk, we highlight the multiple removal mechanisms for infectious particles that have no effect on CO₂ concentration, meaning that the inherent risk in two spaces with identical CO₂ concentrations could vary by an order of magnitude or more depending on which disinfection technologies are deployed. Our overall goal is to illustrate the right way to integrate CO₂ measurements into the overall management of IAQ and airborne transmission risk.