Coronavirus in the air? Only where there are gatherings

The rapid spread of Covid-19, and the generation of outbreaks of different intensity in different regions of the same country, have raised important questions about the transmission mechanisms of the virus and on the role of airborne transmission via respiratory droplets. While the transmission of SARS-CoV-2 by contact (direct or indirect via contact surfaces) is widely accepted, airborne transmission is still a matter of debate in the scientific community.

A multidisciplinary study conducted by the Institute of Atmospheric and Climate Sciences of the National Research Council (CNR-ISAC) of Lecce, Ca’ Foscari University of Venice, the Institute of Polar Sciences of the CNR (CNR-ISP) of Venice and the Experimental Zooprophylactic Institute of Puglia and Basilicata (IZSPB), has analysed the concentrations and size distributions of the viral particles in outdoor air collected simultaneously, during the pandemic, in Veneto and Puglia in the month of May 2020, between the end of the lockdown and the reopening of activities.

The research, launched thanks to the project “AIR-CoV” (Evaluation of the concentration and size distribution of SARS-CoV-2 in air in outdoor environments) and published in the scientific journal Environment International, revealed a low probability of airborne transmission of the virus outdoors except in the places where people gather.

“Our study examined two cities with different rates of infection: Venice-Mestre and Lecce, located in different parts of the country (Northern and Southern Italy) characterized by very different rates of COVID-19 infection in the first phase of the pandemic,” explains Daniele Contini, researcher at CNR-ISAC.

During the first phase of the pandemic, the spread of SARS-CoV-2 was exceptionally severe in Veneto, with a maximum of 10,800 active cases (i.e. infected people) as at 16 April 2020 (about 10% of the total cases in Italy) in a population of 4.9 million. However, Puglia reached the maximum number active cases on 3 May 2020 with 2,955 cases (3% of the total cases in Italy) out of a population of 4.0 million. At the beginning of the measurement period (13 May 2020), Veneto and Puglia had, respectively, 5,020 and 2,322 active cases.

“The role of airborne transmission depends on several variables, such as the concentration and size distribution of viral particles in the atmosphere and the weather conditions. These variables then differ according to whether we consider outdoor or indoor environments,” adds Marianna Conte, researcher at CNR-ISAC.

The potential existence of the SARS-CoV-2 virus in the aerosol samples analysed was determined by collecting the atmospheric particulate of different sizes, from nanoparticles to PM10, and using advanced laboratory diagnostics to determine the presence of the genetic material (RNA) of SARS-CoV-2.

“All the samples collected in residential and urban areas in both cities were negative: the concentration of viral particles was very low in PM10 (less than 0.8 copies per m3 of air) and in each size range analysed (less than 0.4 copies/m3 of air),” continues Contini. “Therefore, the probability of airborne transmission of the virus outdoors, with the exclusion of very crowded areas, appears very low, almost negligible. In gatherings, there can be an local increase in concentrations as well as the risk of close contact, so it is absolutely essential to comply with the rules against gathering even outdoors.”

“The risk could be greater in poorly ventilated indoor community environments, where the smallest respiratory droplets can remain in suspension for longer and even deposit on surfaces,” emphasises Andrea Gambaro, professor at Ca’ Foscari. “Therefore, it is advised to mitigate risk by means of regular ventilation of rooms, sanitising hands and surfaces and using face masks.”

“Today, the study and application of sensitive analytical methods with the use of technologically advanced platforms allows us to detect the presence of Sars-CoV-2 even at very low concentrations, as could be the situation in outdoor and indoor environments, making laboratory diagnostics increasingly reliable,” concludes Giovanna La Salandra, manager of the Research and Scientific Development Department of the IZSPB.
The study of concentrations in certain indoor community environments will be the subject of a second phase of the AIR-CoV project.