While a global pandemic by SARS-CoV in 2002–2003 was averted by fast identification, effective surveillance, and quarantine, such measures cannot be easily transferred to new situations, as the case of SARS-CoV-2 demonstrates. Knowledge of the fundamental biological and physical parameters affecting transmission pathways of respiratory pathogens is thus critical for the design of effective non-pharmaceutical intervention strategies. Respiratory pathogens are transmitted via respiratory droplets containing lung fluid laden with infectious pathogens. The generation, transport and eventual fate (deposition, inhalation) of respiratory droplets are key processes in the transmission pathways of respiratory pathogens. Respiratory droplets are generated within the human respiratory tract, thoracic or extrathoracic, with possibly different pathogen loads, or upon release from an infected person via lung-fluid fragmentation (Bourouiba, Dehandschoewercker, and Bush 2014). They are expelled by expiratory events that include violent events such as coughing or sneezing and quiescent ones such as talking, breathing, or laughing. Respiratory droplets have been associated with three modes of pathogen transmission: in the medical literature, these are referred to as “contact,” “droplet,” and “airborne” transmission modes (Weber and Stilianakis 2008a). Contact transmission, be it direct or indirect, occurs via contact with pathogen-laden droplets: transfer of pathogens via physical touch between a susceptible and an infected host (e.g., hand contact) is classified as direct contact transmission, whereas transfer mediated by fomites containing settled droplets is classified as indirect contact transmission. Droplet transmission refers to transmission by large droplets (diameter dp > 20 microns) that are transported by the turbulent air flow generated by a violent expiratory event. They are, subsequently, sprayed and directly deposited upon the conjunctiva or mucus membranes of a susceptible host. Since large droplets gravitationally settle rather quickly, droplet transmission is considered important at close range: in still air, a 50-micron droplet crosses a vertical 1.5m distance in 20 s (Drossinos and Housiadas 2006). Airborne transmission, also referred to as “aerosol transmission,” refers to pathogen transmission via inhalation of small respiratory droplets (typically smaller than 10 microns: a 10-micron droplet settles gravitationally in still air within approximately 9min). Being relatively small they may deposit deep into the respiratory tract, including the alveolar region. These droplets, often referred to by the confusing term “droplet nuclei,” are small enough to remain airborne for sufficient time to transmit the pathogen. Hence, airborne transmission does not require direct face-to-face contact. The demarcation between the three transmission modes is not clearly specified, as it is not based on welldefined droplet physical properties or their dynamics, often creating confusion. For example, droplets associated with droplet transmission may be transported by a turbulent jet and subsequently inhaled. The classification of transmission modes depends on the size of the droplets. The size distribution of expelled droplets has been a subject of considerable research and controversy (partly attributable to different instrumentation or collection methods). Nevertheless, it is reasonable to consider that respiratory-droplet diameters vary from 0.5 microns to 1000 microns (Duguid 1946; Loudon and Roberts 1967, Papineni and Rosenthal 1997; Chao et al. 2009; Morawska et al. 2009; Asadi et al. 2019). Care should be exercised in interpreting droplet sizes. Respiratory droplets are generated in a nearly 100% relative-humidity environment. Upon exhalation into the lower-humidity ambient environment they shrink by evaporation (a fast molecular process, of the order of seconds or less, depending on droplet size, composition, and relative humidity) to reach their equilibrium diameter. Some estimates suggest that droplets may shrink to about half their original size (Nicas, Nazaroff, and Hubbard 2005, Parienta et al. 2011). Not all pathogen transmission modes are relevant for all respiratory infections. The dominant transmission mode will depend on the interplay of a number of factors, including frequency of violent droplet-generating events (coughing, sneezing), droplet size distribution, ambient relative humidity, viral load, virus inactivation rates, deposition location of inhaled droplets in the airway, and infectious dose. The identification of the dominant transmission mode is essential for a proper and efficient strategy to control the spread of an epidemic, including the proper choice of personal protective equipment. The companion Aerosol Science and Technology Editorial (Asadi et al. 2020) argues cogently for the

中文翻译：

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气溶胶物理学告诉我们有关空气传播病原体的什么信息

虽然通过快速识别、有效监测和隔离避免了 2002 年至 2003 年 SARS-CoV 的全球大流行，但这些措施不能轻易转移到新的情况中，正如 SARS-CoV-2 的案例所证明的那样。因此，了解影响呼吸道病原体传播途径的基本生物学和物理参数对于设计有效的非药物干预策略至关重要。呼吸道病原体通过含有充满传染性病原体的肺液的呼吸道飞沫传播。呼吸道飞沫的产生、运输和最终归宿（沉积、吸入）是呼吸道病原体传播途径中的关键过程。呼吸道飞沫产生于人体呼吸道、胸腔或胸腔外，可能具有不同的病原体载量，或通过肺液碎裂从感染者身上释放（Bourouiba、Dehandschoewercker 和 Bush 2014）。他们被呼气事件排出体外，包括剧烈事件（如咳嗽或打喷嚏）和静止事件（如说话、呼吸或大笑）。呼吸道飞沫与三种病原体传播模式有关：在医学文献中，这些被称为“接触”、“飞沫”和“空气传播”传播模式（Weber 和 Stilianakis 2008a）。接触传播，无论是直接的还是间接的，都是通过接触载有病原体的飞沫而发生的：病原体通过易感宿主和受感染宿主之间的身体接触（例如手接触）传播被归类为直接接触传播，而传播则由含有病原体的污染物介导沉降的飞沫被归类为间接接触传播。飞沫传播是指通过剧烈呼气事件产生的湍流气流传输的大飞沫（直径 dp > 20 微米）的传播。随后，它们被喷洒并直接沉积在易感宿主的结膜或粘膜上。由于大液滴在重力作用下沉降得相当快，因此认为近距离的液滴传播很重要：在静止的空气中，50 微米的液滴在 20 秒内穿过 1.5 米的垂直距离（Drossinos 和 Housiadas 2006）。空气传播，也称为“气溶胶传播”，是指病原体通过吸入呼吸道小飞沫（通常小于 10 微米：10 微米的飞沫在静止空气中重力沉降约 9 分钟内）进行传播。由于相对较小，它们可能会沉积在呼吸道深处，包括肺泡区域。这些飞沫通常被称为“飞沫核”，它们足够小，可以在空气中停留足够长的时间来传播病原体。因此，空气传播不需要直接的面对面接触。三种传输模式之间的界限没有明确规定，因为它不是基于明确定义的液滴物理特性或其动力学，通常会造成混淆。例如，与液滴传输相关的液滴可以通过湍流射流传输并随后被吸入。传输模式的分类取决于液滴的大小。被排出液滴的尺寸分布一直是大量研究和争议的主题（部分归因于不同的仪器或收集方法）。尽管如此，认为呼吸道飞沫直径从 0.5 微米到 1000 微米不等是合理的（Duguid 1946；Loudon 和 Roberts 1967，Papineni 和 Rosenthal 1997；Chao 等人 2009；Morawska 等人 2009；Asadi 等人 2019 年） ）。在解释液滴尺寸时应小心谨慎。呼吸道飞沫是在接近 100% 的相对湿度环境中产生的。当呼气到较低湿度的周围环境中时，它们会通过蒸发（一种快速分子过程，几秒钟或更短时间，取决于液滴大小、成分和相对湿度）收缩，以达到它们的平衡直径。一些估计表明，液滴可能会缩小到原来尺寸的一半左右（Nicas、Nazaroff 和 Hubbard 2005，Parienta 等人 2011）。并非所有病原体传播模式都与所有呼吸道感染相关。主要的传播模式将取决于许多因素的相互作用，包括猛烈的飞沫产生事件（咳嗽、打喷嚏）的频率、飞沫大小分布、环境相对湿度、病毒载量、病毒灭活率、吸入飞沫在体内的沉积位置。气道和感染剂量。确定主要传播模式对于控制流行病传播的正确有效策略至关重要，包括正确选择个人防护装备。同伴气溶胶科学与技术社论（Asadi 等人，2020 年）有力地支持 帕连塔等人。2011）。并非所有病原体传播模式都与所有呼吸道感染相关。主要的传播模式将取决于许多因素的相互作用，包括猛烈的飞沫产生事件（咳嗽、打喷嚏）的频率、飞沫大小分布、环境相对湿度、病毒载量、病毒灭活率、吸入飞沫在体内的沉积位置。气道和感染剂量。确定主要传播模式对于控制流行病传播的正确有效策略至关重要，包括正确选择个人防护装备。同伴气溶胶科学与技术社论（Asadi 等人，2020 年）有力地支持 帕连塔等人。2011）。并非所有病原体传播模式都与所有呼吸道感染相关。主要的传播模式将取决于许多因素的相互作用，包括猛烈的飞沫产生事件（咳嗽、打喷嚏）的频率、飞沫大小分布、环境相对湿度、病毒载量、病毒灭活率、吸入飞沫在体内的沉积位置。气道和感染剂量。确定主要传播模式对于控制流行病传播的正确有效策略至关重要，包括正确选择个人防护装备。同伴气溶胶科学与技术社论（Asadi 等人，2020 年）有力地支持 主要的传播模式将取决于许多因素的相互作用，包括猛烈的飞沫产生事件（咳嗽、打喷嚏）的频率、飞沫大小分布、环境相对湿度、病毒载量、病毒灭活率、吸入飞沫在体内的沉积位置。气道和感染剂量。确定主要传播模式对于控制流行病传播的正确有效策略至关重要，包括正确选择个人防护装备。同伴气溶胶科学与技术社论（Asadi 等人，2020 年）有力地支持 主要的传播模式将取决于许多因素的相互作用，包括猛烈的飞沫产生事件（咳嗽、打喷嚏）的频率、飞沫大小分布、环境相对湿度、病毒载量、病毒灭活率、吸入飞沫在体内的沉积位置。气道和感染剂量。确定主要传播模式对于控制流行病传播的正确有效策略至关重要，包括正确选择个人防护装备。同伴气溶胶科学与技术社论（Asadi 等人，2020 年）有力地支持 病毒灭活率、吸入飞沫在气道中的沉积位置和感染剂量。确定主要传播模式对于控制流行病传播的正确有效策略至关重要，包括正确选择个人防护装备。同伴气溶胶科学与技术社论（Asadi 等人，2020 年）有力地支持 病毒灭活率、吸入飞沫在气道中的沉积位置和感染剂量。确定主要传播模式对于控制流行病传播的正确有效策略至关重要，包括正确选择个人防护装备。同伴气溶胶科学与技术社论（Asadi 等人，2020 年）有力地支持