In Environmental Research, a peer-reviewed publication of ScienceDirect, researchers from a number of universities in Italy have shown that the probability of outdoor airborne transmission of COVID-19 is very low. They write (abridged):
The spread of SARS-CoV-2 by contact (direct or indirect) is widely accepted, but the relative importance of airborne transmission is still controversial.
Probability of outdoor airborne transmission depends on several parameters, still rather uncertain: virus-laden aerosol concentrations, viability and lifetime, minimum dose necessary to transmit the disease.
In this work, an estimate of outdoor concentrations in northern Italy (region Lombardian) was performed using a simple box model approach, based on an estimate of respiratory emissions, with a specific focus for the cities of Milan and Bergamo (Italy).
In addition, the probability of interaction of virus-laden aerosol with pre-existing particles of different sizes was investigated. Results indicate very low (<1 RNA copy/m3) average outdoor concentrations in public area, excluding crowded zones, case scenario and assuming a number of infects up to 25% of population.
On average, assuming a number of infects equal to 10% of the population, the time necessary to inspire a quantum (i.e. the dose of airborne droplet nuclei required to cause infection in 63% of susceptible persons) would be 31.5 days in Milan.
Volume 193, February 2021, 110603