Measurements of SARS-CoV-2 RNA concentrations in indoor and outdoor air in Italy: implications for the role of airborne transmission

Since the beginning of COVID-19 pandemic there have been scientific questions regarding the role of airborne transmission of SARS-CoV-2 and its influence on COVID-19 spread. One of the key parameters to ascertain the risks of airborne transmission is the concentrations of SARS-CoV-2 genetic material (RNA) in air in different environments. This work investigates this aspect in both outdoor and indoor environments in different towns of Italy during the first and the second wave of pandemic. Outdoor samples were collected using both PM₁₀ low-volume (2.3 m³/h) samplers for 48h in parallel in Venice (Veneto region, north-east of Italy) and Lecce (Puglia region, south-east of Italy) during the first wave of pandemic. In addition, two samples for each site were collected using MOUDI multistage impactors (1.8 m³/h for six days sampling) able to separate 12 size ranges of particles. During the second wave of pandemic in Italy (November-December 2020), air samples were collected in different community indoor environments: one train station and two food markets in metropolitan city of Venice (Veneto Region); one canteen in Bologna (Emilia-Romagna region, central Italy); one shopping centre, one hair salon, and one pharmacy in Lecce (Puglia region). All collected samples were analysed to individuate the presence of SARS-CoV-2 RNA traces using two independent approaches: real-time RT-PCR and ddPCR (Chirizzi et al., 2021). Recovery tests done using Mengo virus strain MC₀ indicate a recovery of about 50%. Modelling of expected average concentrations in the different environments, were also done using influx of customers data and local epidemiological information to compare with measurements. Results show a negligible risk for airborne transmission in residential outdoor zones, excluding crowed areas. In the community indoor environments investigated, indoor ventilation seems to play an important role in reducing concentrations of virus-laden particles as well as the use of facemasks.