Safe CO threshold limits for indoor long-range airborne transmission control of COVID-19.

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