Exhaled CO₂ as a COVID-19 Infection Risk Proxy for Different Indoor Environments and Activities

CO₂ is co-exhaled with aerosols containing SARS-CoV-2 by COVID-19-infected people and can be used as a proxy of SARS-CoV-2 concentrations indoors. Indoor CO₂ measurements by low-cost sensors hold promise for mass monitoring of indoor aerosol transmission risk for COVID-19 and other respiratory diseases. We derive analytical expressions of CO₂-based risk proxies and apply them to various typical indoor environments. The relative infection risk in a given environment scales with excess CO₂ level, and thus, keeping CO₂ as low as feasible in a space allows optimization of the protection provided by ventilation. We show that the CO₂ level corresponding to a given absolute infection risk varies by >2 orders of magnitude for different environments and activities. Although large uncertainties, mainly from virus exhalation rates, are still associated with infection risk estimates, our study provides more specific and practical recommendations for low-cost CO₂-based indoor infection risk monitoring.