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

Abstract

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 Lombardia) 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, 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 (range 2.7-91 days) and 51.2 days in Bergamo (range 4.4-149 days). Therefore, the probability of airborne transmission due to respiratory aerosol is very low in outdoor conditions, even if it could be more relevant for community indoor environments, in which further studies are necessary to investigate the potential risks. We theoretically examined if atmospheric particles can scavenge virus aerosol, through inertial impact, interception, and Brownian diffusion. The probability was very low. 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.

Keywords: Aerosol; Airborne virus transmission; COVID-19; Coagulation; Coronavirus; SARS-CoV-2.

Fig. 1

Map of the region Lombardia with indication of the box model domain. In yellow the area of the city of Milan with the second box domain and in red the location of the city of Bergamo with the third box domain. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 2

Estimated average outdoor concentration as function of the number of infected individuals for the Region Lombardia. WCS is the worst case scenario calculated with larger emissions and lower wind speed and mixing height compared to average. BCS is the best case scenario calculated with larger wind speed and mixing height compared to average.

Fig. 3

Estimated average outdoor concentration as function of the number of infected individuals for the Milan and Bergamo cities. WCS is the worst case scenario calculated with larger emissions and lower wind speed and mixing height compared to average. BCS is the best case scenario calculated with larger wind speed and mixing height compared to average.