水滴在COVID-19疫情中提供了众所周知的传播媒介,颗粒大小与传播途径的分类密切相关。然而,由于不同学科的信息不对称,术语“气溶胶”涵盖了悬浮颗粒的大多数粒径,这可能导致公众在选择流行病预防和控制策略方面的误解。在这次审查中,将患者呼出这些液滴的时间作为初始时间。然后,分析了所有可用的病毒载量和呼出的液滴的数值分布,并将空气中液滴的蒸发模型与呼吸道中液滴核的沉积模型相结合。最后,首次总结了物理传播影响不同大小液滴在不同时间传播风险的观点。结果表明,尽管呼出液滴的分布以小液滴为主,液滴体积与粒径的三次幂成正比,这意味着100μm液滴的病毒载量是初始时间1μm液滴的约106倍。此外,呼出的液滴受到蒸发的传热和传质的影响,水分数,盐浓度,和酸碱平衡(水分数>98%),这导致他们迅速改变,病毒的生存状况也急剧恶化。液滴的初始直径(do)收缩到平衡直径(de,约30%的do)与粒径的二次幂大致成正比,对于1μm的液滴只需要几毫秒,而对于10μm的液滴则需要数百毫秒;换句话说,大液滴携带的病毒可以尽可能地保存。最后,易感人群可能通过不同的随机接触途径吸入感染性液滴核,由于复杂气道中的加速碰撞,较大的液滴核更容易分解为微小颗粒,可以沉积在高风险的肺泡区域。在疾病传播过程中,感染性液滴颗粒大小差异很大,且不同时间节点的传播风险差异较大;因此,模糊术语“气溶胶”不利于分析疾病暴露风险。对疫情防控策略的建议是:1)大飞沫是疾病传播的主要冲突;因此,即使他们最初被自制的面具挡住了,它大大包含了流行病。2)接触的早期阶段,如近距离接触和短距离传输,感染风险最高;因此,社交距离可以有效防止易感人群吸入活性病毒。3)fomite途径的风险取决于与传染性病毒接触的时间;因此,提倡良好的卫生习惯(包括经常洗手,不擦眼,咳嗽礼仪,表面清洁的正常化),虽然盲目和过度的消毒措施是不可取的。4)与大液滴相比,小液滴数量更多,但携带的病毒更少,更容易因蒸发而死亡。
Droplets provide a well-known transmission media in the COVID-19 epidemic, and the particle size is closely related to the classification of the transmission route. However, the term \"aerosol\" covers most particle sizes of suspended particulates because of information asymmetry in different disciplines, which may lead to misunderstandings in the selection of epidemic prevention and control strategies for the public. In this
review, the time when these droplets are exhaled by a patient was taken as the initial time. Then, all available viral loads and numerical distribution of the exhaled droplets was analyzed, and the evaporation model of droplets in the air was combined with the deposition model of droplet nuclei in the respiratory tract. Lastly, the perspective that physical spread affects the transmission risk of different size droplets at different times was summarized for the first time. The results showed that although the distribution of exhaled droplets was dominated by small droplets, droplet volume was proportional to the third power of particle diameter, meaning that the viral load of a 100 μm droplet was approximately 106 times that of a 1 μm droplet at the initial time. Furthermore, the exhaled droplets are affected by heat and mass transfer of evaporation, water fraction, salt concentration, and acid-base balance (the water fraction > 98%), which lead them to change rapidly, and the viral survival condition also deteriorates dramatically. The time required for the initial diameter (do) of a droplet to shrink to the equilibrium diameter (de, about 30% of do) is approximately proportional to the second power of the particle diameter, taking only a few milliseconds for a 1 μm droplet but hundreds of milliseconds for a 10 μm droplet; in other words, the viruses carried by the large droplets can be preserved as much as possible. Finally, the infectious droplet nuclei maybe inhaled by the susceptible population through different and random contact routes, and the droplet nuclei with larger de decompose more easily into tiny particles on account of the accelerated collision in a complex airway, which can be deposited in the higher risk alveolar region. During disease transmission, the infectious droplet particle size varies widely, and the transmission risk varies significantly at different time nodes; therefore, the fuzzy term \"aerosol\" is not conducive to analyzing disease exposure risk. Recommendations for epidemic prevention and control strategies are: 1) Large droplets are the main conflict in disease transmission; thus, even if they are blocked by a homemade mask initially, it significantly contains the epidemic. 2) The early phase of contact, such as close-contact and short-range transmission, has the highest infection risk; therefore, social distancing can effectively keep the susceptible population from inhaling active viruses. 3) The risk of the fomite route depends on the time in contact with infectious viruses; thus, it is important to promote good health habits (including frequent hand washing, no-eye rubbing, coughing etiquette, normalization of surface cleaning), although blind and excessive disinfection measures are not advisable. 4) Compared with the large droplets, the small droplets have larger numbers but carry fewer viruses and are more prone to die through evaporation.