**1. Introduction**

Regardless of whether vaccination is efficient and effective [1, 2], a good knowledge of the mechanisms underlying infection among people would have helped against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spreading by minimizing lockdown measures. The scientific community has largely investigated how an infected person transmits pathogens to a susceptible person [3]. What is known is that an infected person emits a certain number of droplets. Some of these droplets, defined as infected, carry a certain number of virions or viral particles that can potentially infect target host cells if reached. The literature also shows that the emission of droplets can occur in different ways, for example, through speech, breathing, coughing, and sneezing. What is still not commonly agreed upon is related to the i) number, ii) size distribution, and iii) emission velocity of the potentially infected droplets for each of these emission modes. After the seminal papers of Wells [4] and Duguid [5], many investigators have grappled with the issue showing broad differences in the experimental results [6]. The reasons can be identified in i) a not full understanding of the physics underpinning the formation of respiratory droplets, ii) the absence of a common methodology, being the different experiments performed by using different techniques and under different ambient conditions, iii) the lack of a rigorous presentation of data, which is often not provided, and iv) the natural variability across individuals.

In addition, the conditions of the ambient air in which these droplets move have also to be considered. Knowing this information is essential to studying the mechanisms that govern the motion of potentially infected droplets. Safe distance, defined as the average distance to which, ideally, a negligible infection probability exists, can be identified once the emitted infected droplets' trajectory is known. However, rigorous terminology and definitions are needed to investigate droplets' motion based on the scientific method. To date, instead, a lack of unambiguous and agreed terminology exists. For example, terms commonly used for direct transmissions, such as "*droplets*," "*aerosol*," and "*airborne*," have been often used with various meanings by several authors. Tang et al. [7] highlighted how significant confusion over the definition and application of relevant terms among professionals (i.e., clinicians versus aerosol scientists) and the general public generates problems in mutual understanding. A second example is the lack of a distinction between solid and liquid carriers, i.e., droplets. Without it, the impact of the thermohygrometric conditions of the ambient air, i.e., relative humidity and temperature, is not adequately considered when studying particles' motion and viral load (defined as the number of copies of ribonucleic acid (RNA) detected in a certain volume). Solid particles that carry viral charges on their surface do not change size or shape along their trajectory except in particular environmental conditions, i.e., if they are condensation nuclei. On the other hand, droplets emitted with a certain viral load reduce their volume through evaporation along their trajectory, resulting in the variation of the balance of forces and viral load. Although experimental results demonstrated that a SARS-CoV-2 viral load lower than 100,000 virions per cubic centimeter (cc) corresponds to a negligible risk of infection [8, 9], to date, the infection risk as a function of the infected droplets' viral load along their trajectory has not yet been sufficiently investigated.

This chapter underlines what are the unsolved research questions (RQs) crucial for a full understanding of SARS-CoV-2 spreading routes and relative countermeasures. Nevertheless, this chapter proposes rigorous terminology and reports evidence to minimize the risk of SARS-CoV-2 infection based on the existing knowledge. The study demonstrates why an effective strategy cannot disregard, at least, i) the use of the facial mask, ii) the control of the thermohygrometric conditions of the ambient air, and iii) the maintenance of the safe distance. In particular, the simplified model shows that the facial mask acts as a first physical barrier against the larger droplets, which can reach very high viral loads in the case of incomplete evaporation. On the other hand, the control of the thermohygrometric conditions acts as a second immaterial barrier that guarantees the complete evaporation of the smaller droplets that come out of the mask before reaching any susceptible subject. Once droplets are completely evaporated, the virions are released into the surrounding environment, where the viral load is low enough to be unlikely to infect.
