Table 2.

Main points in the evidence that SARS-CoV-2 can create clusters with PM or spreads via PM.

	Date	Reference	Personal point of view	Meaning
1	March 16, 2020	(Setti et al., 2020c,d)	First hypothesized that the rapid spread of COVID-19 related to PM10 pollution because SARS-CoV-2 can create clusters with the atmospheric particles and be carried and detected on PM10.	Supports the hypothesis. Position papers/hypothesis. Сannot be considered as direct or indirect evidence.
2	March 20, 2020	Società Italiana di Aerosol (2020).	Assessed the statement by SIMA as premature, suggesting caution in the interpretation of correlation as causation.	Denies the hypothesis. A note signed by more than 70 researchers. Сannot be considered as direct or indirect evidence, but indicates the premature nature of the hypothesis.
3	April 10, 2020	Contini and Costabile (2020).	Assessed the statement by SIMA as premature, suggesting caution in the interpretation of correlation as causation.	Denies the hypothesis. Сannot be considered as direct or indirect evidence, but indicates the premature nature of the hypothesis.
4	April 25, 2020	Setti et al. (2020b)	The authors of the hypothesis noted the importance of the hypothesis and the need for further research.	Supports the hypothesis. Editorial paper/hypothesis. Сannot be considered as direct or indirect evidence.
5	May–June 2020	(Bontempi, 2020a,b; Bontempi et al., 2020);	The results show that it is not possible to conclude that the COVID-19 diffusion mechanism also occurs through the air, using PM10 as a carrier.	Denies the hypothesis. Statistical studies. Сannot be considered as direct evidence, but indicates the premature nature of the hypothesis and provides indirect evidence.
6	May 26, 2020	Setti et al. (2020a)	“This is the first evidence that SARS-CoV-2 RNA can be present on outdoor particulate matter, thus suggesting that, in conditions of atmospheric stability and high concentrations of PM, SARS-CoV-2 could create clusters with outdoor PM and – by reducing their diffusion coefficient – enhance the persistence of the virus in the atmosphere.”	Supports the hypothesis. Setti et al. (2020a) found SARS-CoV-2 RNA in PM10 daily filters; concentrations of virus-laden particles were not evaluated. The findings in this work cannot be considered as direct or even indirect evidence (see the discussion above).
7	Sept., 2020	Setti et al. (2020e)	Based on a statistical approach, Setti et al. (2020e) expanded their hypotheses.	Supports the hypothesis. Statistical study. Сannot be considered as direct or indirect evidence.
8	Dec. 8, 2020	Belosi et al. (2021)	Found that the probability of coagulation of virus-laden aerosol with pre-existing PM was negligible for micron and submicron particles (>100 nm).	Denies the hypothesis. Theoretical work. Сannot be considered as direct evidence, but indicates the premature nature of the hypothesis and provides indirect evidence.
9	Nov. 12, 2020; Feb. 17, 2021; Apr. 16, 2021	(Chirizzi et al., 2021; Linillos-Pradillo et al., 2021; Pivato et al., 2021)	Did not indicate the presence of SARS-CoV-2 RNA in the PM samples	Сannot be considered as direct evidence, but indicates the premature nature of the hypothesis and provide indirect evidence.
10	April 16, 2021	Rowe et al. (2021)	Showed quantitatively that in most cases, the outdoor risk is orders of magnitude less than the indoor risk.	Theoretical work. Сannot be considered as direct evidence, but indicates the premature nature of the hypothesis and provides indirect evidence.
11	May 25, 2021	Kayalar et al. (2021)	“We demonstrated that SARS-CoV-2 RNA can be present on ambient PM suggesting that the virus may be transported via PM pollution.”	Supports the hypothesis. The methodological part of this work is similar to the study by Setti et al. (2020a). The findings of this work cannot be considered as direct or even indirect evidence (see the discussion above).