Indoor contagion risk strongly reduced thanks to rules

In Italy, the rapid spread of Covid-19 in autumn 2020 — i.e., the second wave of the pandemic — caused the Government to introduce restrictive measures on a regional basis, using a classification of risk expressed with a colour scale. In order to define the risk of contagion with extreme precision, it is crucial that we answer important questions regarding airborne transmission in certain indoor community settings, such as supermarkets, restaurants, and public transport.

"The role of airborne transmission depends on several variables, including the concentration of viral particles, which has been studied mainly in hospitals or for the care of Covid-19 patients," explains Daniele Contini, senior researcher at the Institute of Atmospheric Sciences and Climate (CNR-ISAC)  of Lecce. “However, data regarding indoor community environments is still scarce. For this reason, and for the complexity of the subject, we conducted a study in various Italian cities.”

The research was conducted between November and December 2020, during the maximum spread of the second wave of the pandemic in Italy. The researchers analysed the concentration of viral particles in the air in different community environments which remained active during the restrictions — the railway station in Mestre and two supermarkets in the metropolitan area of ​​Venice; the canteen of the CNR centre of ​​Bologna; a shopping centre, a pharmacy, and a hairdressing salon in Lecce. The data was collected in areas of the country which were significantly different both in terms of the spread of the virus and of atmospheric conditions.

The results were published in the scientific journal Environmental Science and Pollution Research and signed not only by CNR-ISAC, but also by the CNR Institute of Polar Sciences, Ca’ Foscari University of Venice, and Istituto zooprofilattico sperimentale della Puglia e della Basilicata (IZSPB).

"The presence of the virus in aerosol samples was verified by collecting atmospheric particles, PM10 samples and total suspended particle (TSP) samples. By using advanced laboratory techniques, the researchers were able to test for the presence of the genetic material (RNA) of Sars-CoV-2,” says Contini. “All the samples collected were negative and no differences were observed with relation to variables such as opening hours, the presence of people and indoor environments. This means that the virus is either absent or present in a concentration lower than the detection limits of the protocols adopted for the study. These findings seem to indicate that, when restrictions are in place (physical distancing, entry restrictions and the use of masks), the probability of airborne contagion is very low."

"These results are compatible with those of the simulations carried out taking into account the epidemiological situation in the various areas examined. The research highlights the important role of ventilation in indoor environments and of the use of masks in reducing the risk of airborne virus transmission,” explains Franco Belosi, senior researcher at CNR-ISAC. "This reminds us of the importance of observing the rules on masks and social distancing, and it should encourage us to increase ventilation as much as possible."

"Risk is likely to be greater in poorly ventilated indoor environments, where respiratory droplets can remain suspended for longer and settle on surfaces, thereby increasing the possibility of contagion by indirect contact compared to direct contact between individuals,” confirms Andrea Gambaro, lecturer at Ca’ Foscari University of Venice.

"This study also highlights the importance of developing a standard protocol for the assessment of the presence of Sars-CoV-2 in the air, in order to improve detection limits and homogenise the results of different studies," says Giovanna La Salandra, senior biologist at Istituto zooprofilattico sperimentale della Puglia e della Basilicata (IZSPB).