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Airborne transmission of SARS-CoV-2 has been object of debate in the scientific community since the beginning of COVID-19 pandemic. This mechanism of transmission could arise from virus-laden aerosol released by infected individuals and it is influenced by several factors. Among these, the concentration and size distribution of virus-laden particles play an important role. The knowledge regarding aerosol transmission increases as new evidence is collected in different studies, even if it is not yet available a standard protocol regarding air sampling and analysis, which can create difficulties in the interpretation and application of results. This work reports a systematic review of current knowledge gained by 73 published papers on experimental determination of SARS-CoV-2 RNA in air comparing different environments: outdoors, indoor hospitals and healthcare settings, and public community indoors. Selected papers furnished 77 datasets: outdoor studies (9/77, 11.7%) and indoor studies (68/77. 88.3%). The indoor datasets in hospitals were the vast majority (58/68, 85.3%), and the remaining (10/68, 14.7%) were classified as community indoors. The fraction of studies having positive samples, as well as positivity rates (i.e. ratios between positive and total samples) are significantly larger in hospitals compared to the other typologies of sites. Contamination of surfaces was more frequent (in indoor datasets) compared to contamination of air samples; however, the average positivity rate was lower compared to that of air. Concentrations of SARS-CoV-2 RNA in air were highly variables and, on average, lower in outdoors compared to indoors. Among indoors, concentrations in community indoors appear to be lower than those in hospitals and healthcare settings.

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A field campaign was performed simultaneously at five measurement sites, having different characteristics, to characterize the spatial distribution of the carbonaceous content in atmospheric aerosol in Southern Italy during the winter season. Organic carbon (OC) and elemental carbon (EC) were measured at urban (Naples), suburban (Lecce), coastal/marine (Lamezia Terme and Capo Granitola), and remote (Monte Curcio) locations. OC and EC mass concentrations were quantified by the thermal-optical transmission (TOT) method, in 24-h PM 10 and PM 2.5 samples collected on quartz fiber filters, from 25 November 2015 to 1 January 2016. The different sites showed marked differences in the average concentrations of both carbonaceous species. Typically, OC average levels (±standard deviation) were higher at the sites of Naples (12.8 ± 5.1 and 11.8 ± 4.6 µg/m 3 ) and Lecce (10.7 ± 5.8 and 9.0 ± 4.7 µg/m 3 ), followed by Lamezia Terme (4.3 ± 2.0 and 4.0 ± 1.9 µg/m 3 ), Capo Granitola (2.3 ± 1.2 and 1.7 ± 1.1 µg/m 3 ), and Monte Curcio (0.9 ± 0.3 and 0.9 ± 0.3 µg/m 3 ) in PM 10 and PM 2.5 , respectively. Similarly, EC average levels (±standard deviation) were higher at the urban sites of Naples (2.3 ± 1.1 and 1.8 ± 0.5 µg/m 3 ) and Lecce (1.5 ± 0.8 and 1.4 ± 0.7 µg/m 3 ), followed by Lamezia Terme (0.6 ± 0.3 and 0.6 ± 0.3 µg/m 3 ), Capo Granitola (0.3 ± 0.3 and 0.3 ± 0.2 µg/m 3 ), and Monte Curcio (0.06 ± 0.04 and 0.05 ± 0.03 µg/m 3 ) in PM 10 and PM 2.5 , respectively. An opposite trend was observed for the OC/EC ratios ranging from 6.4 to 15.9 in PM 10 and from 6.4 to 15.5 in PM 2.5 with lower values in urban sites compared to remote sites. Different OC-EC correlations, 0.36 < R 2 < 0.90, were found in four observation sites. This behavior suggests the contributions of similar sources and common atmospheric processes in both fractions. No correlations were observed between OC and EC at the site of Naples. The average secondary organic carbon (SOC) concentrations, quantified using the minimum OC/EC ratio method, ranged from 0.4 to 7.6 µg/m 3 in PM 10 and from 0.4 to 7.2 µg/m 3 in PM 2.5 , accounting from 37 to 59% of total OC in PM 10 and from 40 to 57% in PM 2.5 with higher percentages in the urban and suburban sites of Naples and Lecce.