2.2. Data Synthesis

According to the World Health Organization (WHO), the transmission of coronavirus infections, including SARS-CoV-2, occurs through droplets, droplets of diameter ≥ 5 μm that originate from the acts of breathing, speaking, coughing and sneezing. Due to their size, the droplets travel in the air for short distances, generally less than one meter, and can directly reach susceptible subjects in the immediate vicinity, as well as settling on objects or surfaces that therefore become a source of spread of the virus. In fact, in this case, hands that have come into contact with the so contaminated objects can constitute a vehicle of transmission by indirect contact when they touch the mouth, nose and eyes. Given that hand washing is always the cornerstone of correct prevention, regular cleaning followed by disinfection of surfaces and internal environments play a crucial role in preventing and containing the spread of the virus. Studies on coronaviruses, not SARS-CoV-2, such as the SARS and MERS virus, suggest that the survival time of these pathogens on surfaces, in experimental conditions, varies from a few hours to a few days depending on the material involved, concentration, temperature and humidity. It should be emphasized that this data refers to the finding of the virus RNA and not to its isolation in vital form, and therefore not related to its real infectivity ^{[7][8][9][10][11][12][13]}[16,25,26,39,40,41,42].

2.2.1. Coronaviruses in Different Temperatures and Humidity

A recent study of Chin et al. ^[4] [33] assessed the stability of the SARS-CoV-2 virus at different temperatures, showing that the virus is highly stable at 4 °C, but sensitive to heat. In fact, at 4 °C there was a reduction of about 0.7 logarithmic units of the viral titre on the 14th day. By increasing the incubation temperature to 56 °C, a significant decrease in viral infectivity was observed within 10 min and, after 30 min, the virus was no longer detectable. Raising the temperature to 70 °C, the virus was no longer detectable after 5 min. The stability of the SARS-CoV-2 virus on different surfaces was also assessed in the same study. The viral titer on each surface was determined after 30 min, 3 h, 6 h, 1 day, 2 days, 4 days and 7 days of incubation, as illustrated in Table 12, Table 23 and Table 34 (Figure 1). The real novelty of this study is that persistence on some surfaces, such as the external surface of the masks, can last up to seven days ^[14][43]. Different pH seems to not statistically influence the virus persistence (Table 34). The factors influencing the persistence of microorganisms in the air/aerosol can be defined as:

Figure 1. SARS-CoV-2 stability at different temperature over time. TCID₅₀ on y axis and time on x axis (0 is equal to “not detectable” virus (≤10³ TCID₅₀/mL)).