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. 2022 Feb 10;210:112921. doi: 10.1016/j.envres.2022.112921

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Fig. 6 — The figure summarizes the current knowledge on the possible role of atmospheric conditions (High and Low Temperature, High and Low Relative Humidity and Solar Radiation) and of air pollutants (Particulate Matter) as well as of host's defense mechanisms on the fitness and infectious capability of SARS-CoV-2. Our analysis refers to the period, ranging from 18 May to 6 November 2020, in several regions of North Italy. Each of the above-mentioned factors may exert an its own individual activity or it may establish reciprocal interactions with the other climate-, environmental-parameters and with host's defense functions and produce a wide spectrum of effects. Experimental evidence suggests that: A) High temperature, as a single factor or together with Solar Radiation, may reduce viability and survival of SARS-CoV-2 through a direct anti-viral effect probably by: a) decreasing the stability of virions, b) preserving the normal function of ciliate cells, preventing their damage by airborne pathogens and particulate matter, c) affecting desiccation or hydration of viral droplets, modulating the size of droplets in cooperation with Relative Humidity (see also D and E); B) Solar Radiation, alone or in cooperation with environmental High Temperature, displays an antiviral function. Solar UV-B rays may impair the stability of viral capsids by: a) directly reducing the stability of SARS-CoV-2 virions and decreasing their fitness and survival b) indirectly promoting an increase in vitamin D synthesis. This fat-soluble micronutrient regulates the normal activity of different components and elements, belonging to immune system. An adequate antiviral response by host requires a cooperative interplay among these elements, such as cells (lymphocytes/macrophages) and mediators (chemokines, interleukins and oxygen species). Vitamin D3 may contribute to restore a proper function of both innate and adaptive arms of immune system. In particular, SARS-CoV-2 and particulate matter may impair the functionality of both lymphocytes and macrophages, via different mechanisms. Vitamin D3 may counteract the harmful effects caused by virus and by air pollutants and it may promote the reactivation of an adequate antiviral response by lymphocytes and macrophages in the host; C) Low Temperature as a single factor may promote SARS-CoV-2 infectious capability and its survival by: a) impairing host's defences in respiratory tract, via the damage of barrier system (the mucus layer, the surface liquid layer and the cilia on the surface of the bronchus epithelia) as, in normal conditions, these factors are able to counteract virus entry, or via the alteration of innate and adaptive immune response (the network of interferons, macrophages, lymphocytes and interleukins/cytokines) as these elements may block the virus which has by-passed the host's barrier system (b) affecting the physical characteristics of droplets, which carry the virus. In particular, Low Temperature alone or in cooperation with Low or High Relative Humidity may modulate the size of droplets, carrying SARS-CoV-2 and therefore it may influence the capability of this pathogen to infect the host (see also D and E); D) Low and E) High Relative Humidity as single factors may promote the desiccation or hydration of droplets, carrying SARS-CoV-2, and therefore they induce a reduction or an increase in their sizes. These events may influence the infectious capability of the virus, but the data available in literature are not univocal. In particular, data emerging from our analysis indicate that High Relative Humidity is associated with low rate of hospital admission. The role of Relative Humidity is debated and represents a proper example of how the mutual interplay among atmospheric, environmental and host's factors impacts on the virus spreading and on human health. In particular, it is possible that the size of droplets may exert a critical role in influencing the capability of SARS-CoV-2 to spread in the environment and to infect the host. It is probable that Low Humidity itself decreases viral viability and alone or with the concomitant presence of elevated temperature induces the formation of droplets containing viral particles with a reduced fitness and having suboptimal sizes for the spreading in the environment and for the entry in host's respiratory tract. On the other hand, High Relative Humidity may increase SARS-CoV-2 fitness and infectious capability, but, even in presence of low temperature, it may promote the generation of bigger droplets, but having sizes not adequate for a proper diffusion and infectivity of the virus. All these considerations may contribute to explain the not univocal data detectable in literature, concerning this topic. It is possible that the final effects depend on the overall balance among all these interactions. F) Particulate matter may affect SARS-CoV-2 infectivity and spreading, by: a) impairing several host's functions. The most important alterations induced by air pollution include a decrease in macrophage and lymphocyte activity, a reduction of antioxidant cell systems as well as an increase of protease generation. Furthermore, particulate matter may increase RAAS activation. A further possible effect of particulate matter is to act as carrier of droplets. RAAS: Renin-Angiotensin-Aldosterone-System; ROS: Reactive Oxygen Species generation; SARS-CoV-2: Severe-Acute-Respiratory-Syndrome associated with Coronavirus 2. Green arrows indicate protective antiviral actions, mediated by environmental- and atmospheric-factors as well as elicited by host's defensive mechanisms. Red and black arrows indicate harmful viral activities, induced by environmental- and atmospheric-factors as well as elicited by host's responses. Green lines with flat termination indicate inhibitory actions of environmental- and atmospheric-factors as well as of host's responses against SARS-CoV-2 with protective antiviral effects. Red lines with flat termination indicate harmful actions of SARS-CoV-2, inhibiting host's protective antiviral functions. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)