The unprecedented pandemic of the COVID-19 poses a threat to public health worldwide. There is widespread agreement between members of the infectious disease community, about possible methods of transmitting the respiratory virus (Tellier et al., 2019; Morawska and Cao, 2020). A person becomes susceptible to infection through “direct” or “indirect” physical contact with the touch of a virus-infected hand. Direct contact indicates that the virus is transmitted through person-to-person contact (such as handshake) between an infected host and a susceptible person. While indirect contact means transmission through “fomite”, which is caused by contact with an object such as a paper tissue or hand-rail that has been infected with a virus. On the other hand, airborne transmission can occur through two different modes and does not require physical contact between an infected person and a susceptible person (Lindsley et al., 2013; Bourouiba, 2020). During sneezing or coughing, a “droplet spray” of virus-laden airway fluid, usually more than 5 μm in diameter, directly affects a susceptible person. Alternatively, a susceptible individual can inhale microscopic particles and aerosol evaporated from the remaining solid components of the respiratory aerosol, which are droplets tiny enough (<5 μm) to stay airborne for hours. That is not clear which mechanisms play a major role in COVID-19 transmission. Much research on airborne diseases before the COVID-19 has focused on “exhalation” events such as coughing and sneezing (Lindsley et al., 2013; Bourouiba et al., 2014; Dhand and Li, 2020). There is now strong evidence; although many infected people who transmit the COVID-19 either at least symptomatic or are not at all symptomatic. In China, asymptomatic individuals were found to be positive for the SARS-CoV-2, and transmission of the virus from asymptomatic carriers has been identified (Chan et al., 2020; Zou et al., 2020; Hu et al., 2020; Rothe et al., 2020). Recently, epidemiologists have estimated that more than 85 % of infections in China (Wuhan), were “undocumented” prior to the travel restrictions, with “limited, mild or no symptoms” and never they were tested. Remarkably, the modeling shows that 79 % of cases are actually infected by undocumented peoples. In addition, inspecting of average delay timing between initial manifestations of symptoms and infection indicates that ” … pre-symptomatic [virus] shedding in some cases may be typically documented” (Li et al., 2020; Yongjian et al., 2020). In fact, it seems that a large number of patients who are sufficiently sick and in need of hospital treatment may themselves have been infected by people who did not appear to be ill. By definition, asymptomatic people do not have a significant cough or sneeze (Dhand and Li, 2020; Lednicky et al., 2020; Kenarkoohi et al., 2020). Indirect and or direct contact modes and also the transfer of aerosol are the main possible transmission modes. Much of public health messages have rightly focused on the possibility of indirect and or direct transmission through contaminated hands, and focus on the importance of greeting without shaking hands with others and washing hands thoroughly. There are important reasons to believe that aerosols play a role in high COVID-19 transmissibility (Matson et al., 2020; Booth et al., 2005). In the 2003 epidemic, air sampling showed that the viable aerosolized virus was emitted into the air by hospitalized SARS patients (Booth et al., 2005; Tang et al., 2020). The SARS-CoV-1 virus is the closest known in humans to SARS-CoV-2 which is responsible for the COVID-19 pandemic. Recent experiments have shown that the SARS-CoV-2 aerosol remains viable in the air with a 1-h half-life. As a result, SARS-CoV-2 aerosol and fomite transmission are acceptable because the virus may remain to survive and infectious in the aerosols and on the surfaces for hours and up to days (Van Doremalen et al., 2020). In addition to coughing and sneezing, has been shown that both breathing and normal speech emit large amounts of aerosol particles (Duguid, 1946; Papineni and Rosenthal, 1997). Most people who are unfamiliar with aerosols particles are completely unaware of their existence, these exhaled particles are typically diameter about 1 μm and therefore invisible by the naked eye. These particles are large enough to carry the SARS-CoV-2 virus, and also they are within the correct range of size and can be easily inhaled deep into a susceptible person’s respiratory tract (Yongjian et al., 2020; Tang et al., 2020; Heyder et al., 1986).
Recent studies on viral respiratory disease (such as influenza) have shown that a viable virus can be emitted from infected peoples by speaking even breathing, without sneezing or coughing (Dhand and Li, 2020; Yan et al., 2018). Normal and ordinary speech converts significant amounts of respiratory particles into airborne aerosols. Experimental research has shown that vocalization emits up more aerosols than breathing (Morawska et al., 2009), also, a recent study indicated the louder one speech, the more aerosols are produced (Asadi et al., 2019). COVID-19 is a severe respiratory infection, and recent studies clearly identified the presence of the SARS-CoV-2 in a tract of the respiratory system (Zhu et al., 2019). So particles derived from breath and speech may contain viruses. These particles may be due in part to the mechanism of “liquid film bursting” in alveoli in the pulmonary, and or through the vibration of the vocal cords during a speech (Johnson et al., 2011). The findings suggest that particles and aerosols in the air reach the brain and affect CNS health (Ehsanifar et al., 2021), with changes in the blood-brain barrier (BBB) or leakage and transmission along the olfactory nerve to the olfactory bulb (OB) and active Microglia are the main components (Ehsanifar et al., 2019a, 2019b, 2020, 2021a, 2021b).
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