Abstract/Summary

CO2-based infection risk monitoring is highly recommended under the current COVID-19 pandemic. However, the monitoring thresholds proposed in the literature are mainly for spaces with fixed occupants. Determining threshold is challenging in spaces with changing occupancy due to the co-existence of quanta and remaining from the previous occupants. Here, we propose a new calculation framework to derive safe excess thresholds (above outdoor level), , for various spaces with fixed/changing occupancy and analyze the uncertainty entailed. Common indoor spaces were categorized into three scenarios according to their occupancy condition, e.g., fixed or varying infection ratios (infectors/occupants). We proved that rebreathed fraction-based model can be directly applied for Ct derivation in the cases of a fixed infection ratio (Scenario 1 and Scenario 2). In the case of varying infector ratios (Scenario 3), Ct derivation has to follow the general calculation framework due to the existence of initial quanta/excess CO2. Otherwise, significant bias can be caused for Ct (e.g., 260 ppm) when infection ratio varies remarkably. Ct significantly varies with specific space factors such as occupant number, activities, and community prevalence, e.g., 7 ppm for gym and 890 ppm for lecture hall, indicating Ct should be determined on a case-by-case basis. An uncertainty of Ct up to 6 orders of magnitude was found for all cases due to uncertainty in emissions of quanta and CO2, thus emphasizing the role of accurate emissions data in obtaining Ct.