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Preventive measures to reduce infection are needed to combat the COVID-19pandemic and prepare for a possible endemic phase. Current prophylacticvaccines are highly effective to prevent disease but lose their ability toreduce viral transmission as viral evolution leads to increasing immune escape.Long term proactive public health policies must therefore complementvaccination with available nonpharmaceutical interventions (NPI) aiming toreduce the viral transmission risk in public spaces. Here, we revisit thequantitative assessment of airborne transmission risk, considering asymptoticlimits that considerably simplify its expression. We show that the aerosoltransmission risk is the product of three factors: a biological factor thatdepends on the viral strain, a hydrodynamical factor defined as the ratio ofconcentration in viral particles between inhaled and exhaled air, and a facemask filtering factor. The short range contribution to the risk, both presentindoors and outdoors, is related to the turbulent dispersion of exhaledaerosols by air drafts and by convection (indoor), or by the wind (outdoors).We show experimentally that airborne droplets and CO$_2$ molecules present thesame dispersion. As a consequence, the dilution factor, and therefore the risk,can be measured quantitatively using the CO$_2$ concentration, regardless ofthe room volume, the flow rate of fresh air and the occupancy. We show that thedispersion cone leads to a concentration in viral particles, and therefore ashort range transmission risk, inversely proportional to the squared distanceto an infected person and to the flow velocity. The aerosolization criterionderived as an intermediate result, which compares the Stokes relaxation time tothe Lagrangian time scale, may find application for a broad class ofaerosol-borne pathogens and pollutants.

Risk assessment for long and short range airborne transmission of SARS-CoV-2, indoors and outdoors