The COVID-19 pandemic has highlighted the need for a proper risk assessment of respiratory pathogens in indoor settings. This paper documents the COVID Airborne Risk Assessment methodology, to assess the potential exposure of airborne SARS-CoV-2 viruses, with an emphasis on virological and immunological factors in the quantification of the risk. The model results from a multidisciplinary approach linking physical, mechanical and biological domains, enabling decision makers or facility managers to assess their indoor setting. The model was benchmarked against clinical data, as well as two real-life outbreaks, showing good agreement. A probability of infection is computed in several everyday-life settings and with various mitigation measures. The importance of super-emitters in airborne transmission is confirmed: 20% of infected hosts can emit approximately two orders of magnitude more viral-containing particles. The use of masks provides a fivefold reduction in viral emissions. Natural ventilation strategies are very effective to decrease the concentration of virions, although periodic venting strategies are not ideal in certain settings. Although vaccination is an effective measure against hospitalization, their effectiveness against transmission is not optimal, hence non-pharmaceutical interventions (ventilation, masks) should be actively supported. We also propose a critical threshold to define an acceptable risk level.

The global crisis triggered by the COVID-19 pandemic has highlighted the need for a proper risk assessment of respiratory pathogens in indoor settings. This paper documents the COVID Airborne Risk Assessment (CARA) methodology, to assess the potential exposure of airborne SARS-CoV-2 viruses, with an emphasis on the effect of certain virological and immunological factors in the quantification of the risk. The proposed model is the result of a multidisciplinary approach linking physical, mechanical and biological domains, benchmarked with clinical and experimental data, enabling decision makers or facility managers to perform risk assessments against airborne transmission. The model was tested against two benchmark outbreaks, showing good agreement. The tool was also applied to several everyday-life settings, in particular for the cases of a shared office, classroom and ski cabin. We found that 20% of infected hosts can emit approximately 2 orders of magnitude more viral-containing particles, suggesting the importance of super-emitters in airborne transmission. The use of surgical-type masks provides a 5-fold reduction in viral emissions. Natural ventilation through the opening of windows at all times are effective strategies to decrease the concentration of virions and slightly opening a window in the winter has approximately the same effect as a full window opening during the summer. Although vaccination is an effective protection measure, non-pharmaceutical interventions, which significantly reduce the viral density in the air (ventilation, masks), should be actively supported and included early in the risk assessment process. We propose a critical threshold value approach which could be used to define an acceptable risk level in a given indoor setting.

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