key: cord-0693597-05bg7rfa authors: Niazi, Sadegh; Groth, Robert; Spann, Kirsten; Johnson, Graham R. title: The role of respiratory droplet physicochemistry in limiting and promoting the airborne transmission of human coronaviruses: A critical review() date: 2020-11-06 journal: Environ Pollut DOI: 10.1016/j.envpol.2020.115767 sha: f005285c7860c36b2b38b04055030991b56572d5 doc_id: 693597 cord_uid: 05bg7rfa Whether virulent human pathogenic coronaviruses (SARS-CoV, MERS-CoV, SARS-CoV-2) are effectively transmitted by aerosols remains contentious. Transmission modes of the novel coronavirus have become a hot topic of research with the importance of airborne transmission controversial due to the many factors that can influence virus transmission. Airborne transmission is an accepted potential route for the spread of some viral infections (measles, chickenpox); however, aerosol features and infectious inoculum vary from one respiratory virus to another. Infectious virus-laden aerosols can be produced by natural human respiratory activities, and their features are vital determinants for virus carriage and transmission. Physicochemical characteristics of infectious respiratory aerosols can influence the efficiency of virus transmission by droplets. This critical review identifies studies reporting instances of infected patients producing airborne human pathogenic coronaviruses, and evidence for the role of physical/chemical characteristics of human-generated droplets in altering embedded viruses’ viability. We also review studies evaluating these viruses in the air, field studies and available evidence about seasonality patterns. Ultimately the literature suggests that a proportion of virulent human coronaviruses can plausibly be transmitted via the air, even though this might vary in different conditions. Evidence exists for respirable-sized airborne droplet nuclei containing viral RNA, although this does not necessarily imply that the virus is transmittable, capable of replicating in a recipient host, or that inoculum is sufficient to initiate infection. However, evidence suggests that coronaviruses can survive in simulated droplet nuclei for a significant time (>24 h). Nevertheless, laboratory nebulized virus-laden aerosols might not accurately model the complexity of human carrier aerosols in studying airborne viral transport. In summary, there is disagreement on whether wild coronaviruses can be transmitted via an airborne path and display seasonal patterns. Further studies are therefore required to provide supporting evidence for the role of airborne transmission and assumed mechanisms underlying seasonality. -Airborne transmission: the spread of a pathogen embedded in an aerosol from a source to a 93 susceptible host, causing infection of the host with or without the consequent disease. (Hinds, 94 1982) 95 -Aerosol: a collection of particles (liquid or solid) ranging in size from 0.001µm to over 100 96 mm suspended in a gas. (Wells, 1934) 97 -Droplet nucleus: The airborne residue (with or without embedded pathogens) of a respiratory 98 droplet containing non-volatile solutes, from which water has evaporated to the point of 99 equilibrium with the ambient relative humidity. (Wells, 1934) The smaller laryngeal and oral mode droplets are small enough to achieve equilibrium with 153 the ambient relative humidity. These droplets therefore form airborne droplet nuclei while the 154 much larger oral mode droplets settle or impact with surfaces before equilibrium can be 155 achieved. 156 The average mass concentration of these modes varies with the respiratory manoeuvre. For concentration in the exhaled aerosol decreases when the volunteer held their breath after 161 inhaling. The consistent findings of Johnson and Holmgren that fewer particles are observed 162 after breath-holding, this would suggest that the generated particles have adequate time to 163 deposit on the airways before being exhaled, consistent with the FFB model proposed by 164 Johnson. Based on previous literature, healthy subjects can produce particles between 0.01 The aerosols generated through speech, coughing, sneezing, and breathing have been 178 surveyed in several studies (Table 1) 290 Hygroscopic salts influence the transport of water vapor, and allow for humidity dependent 359 droplet sizes as described by Köhler theory (Köhler, 1936) . Because of their hygroscopic 360 behavior, inorganic salts such as NaCl can play an essential role in controlling the water 361 uptake and loss when present in an aerosol, and consequently limit or promote virus viability. NaCl is a prominent constituent found in human respiratory aerosol. Some hygroscopic salts, Several studies have assessed environmental samples of wild coronaviruses collected in field 435 settings (Table 5) It is predicted that COVID-19 incidence will align similarly to SARS-CoV, potentially 576 showing seasonality. 577 In this critical review, we reviewed the ability of infected individuals to produce droplet 597 laden-virus with pandemic potential, including SARS-CoV, MERS-CoV, and SARS-CoV-2. There is evidence explaining the production mechanisms of airborne pathogenic bioaerosols 599 through human respiratory activities, which can travel distance over several meters in the air 600 and remain infectious. Studies investigating respiratory droplets and droplet nuclei generation 601 during respiratory activities have shown that the produced droplets are of adequate size to 602 support an infectious virus. Additionally, viral RNA was found in the air of patients' rooms 603 under different conditions. The airborne transmission was thought to play an essential role in 604 the epidemiology of several highly transmissible coronaviruses that emerged this century. 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