Monitoring carbon dioxide to quantify the risk of indoor airborne transmission of COVID-19

Bazant, M. Z.; Kodio, O.; Cohen, A. E.; Khan, K.; Gu, Z.; Bush, J. W. M.

A new guideline for mitigating indoor airborne transmission of COVID-19 prescribes a limit on the time spent in a shared space with an infected individual (Bazant & Bush, 2021). Here, we rephrase this safety guideline in terms of occupancy time and mean exhaled carbon dioxide concentration in an indoor space, thereby enabling the use of $mathrm{CO_2}$ monitors in the risk assessment of airborne transmission of respiratory diseases. While CO2 concentration is related to airborne pathogen concentration (Rudnick & Milton, 2003), the guideline accounts for the different physical processes affecting their evolution, such as enhanced pathogen production from vocal activity and pathogen removal via face-mask use, filtration, sedimentation and deactivation. Critically, transmission risk depends on the total infectious dose, so necessarily depends on both the pathogen concentration and exposure time. The transmission risk is also modulated by the relative magnitudes of susceptible, infected and immune persons within a population, which evolve as the pandemic runs its course. A general mathematical theory is developed to predict airborne transmission risk from CO2 time series in real time and clarify various approximations that lead to the guideline. Illustrative examples of assessing transmission risk and implementing the guideline are presented using data from CO2 monitoring in university classrooms and office spaces.