Study Shows Virus Outdoor Airborne Transmission is Negligible

January 29, 2021

New study shows that in public or residential areas in the outdoors there are no detectable concentration of covid viruses, and the probability of covid outdoor airborne transmission is extremely low, unlike inside settings.

A group of Italian researchers estimating concentrations of the covid virus in outdoor air determined that the probability of covid outdoor airborne transmission is negligible. They found no detectable concentration of SARS-CoV-2 in public or residential areas in the outdoors, unlike inside settings where many infected individuals could be in the same small environment.

In this new study – titled, “On the concentration of SARS-CoV-2 in outdoor air and the interaction with pre-existing atmospheric particles”, revised and published in “Environmental Research – A Multidisciplinary Journal of Environmental Sciences and Engineering” – an estimate of outdoor concentrations in northern Italy (region Lombardia) was performed, with a specific focus for the cities of Milan and Bergamo.

Authors say: “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,” they explain, “an average outdoor SARS-CoV-2 virus-laden aerosol concentrations, due to respiratory emissions of infected individuals in the Lombardia region (Northern Italy, Po valley pollution hot-spot), were investigated as function of the number of infected individuals (including asymptomatic). This was done using three simple box models: one covering all region, the second centred on the city of Milan, and the third centred on the city of Bergamo, where a covid-19 outbreak was observed in March 2020.”

“Results indicate very low (<1 RNA copy/m3) average outdoor concentrations in public area, excluding crowded zones, even in the worst 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 … and 51.2 days in Bergamo…”

“Therefore, the probability of airborne transmission due to respiratory aerosol is very low in outdoor conditions excluding public crowded areas. This transmission mechanism could be more relevant for indoor community environments, in which further studies are necessary to investigate the potential risks.”

“The probability”, researchers conclude, “of the viral particles to be scavenged from atmospheric aerosol particles, due to inertial, interception and Brownian capture mechanisms, was negligible. In addition, the probability of coagulation of virus-laden aerosol with pre-existing atmospheric particles resulted negligible for accumulation and coarse mode particles, but virus-laden aerosol could act as sink of ultrafine particles (around 0.01 μm in diameter). However, this will not change significantly the dynamics behaviour of the virus particle or its permanence time in atmosphere.”

Full Study “On the concentration of SARS-CoV-2 in outdoor air and the interaction with pre-existing atmospheric particles”