To quantify the fate of respiratory droplets under different ambient relative humidities, direct numerical simulations of a typical respiratory event are performed. We found that, because small droplets (with initial diameter of $10\mu \mathrm{m}$) are swept by turbulent eddies in the expelled humid puff, their lifetime gets extended by a factor of more than 30 times as compared to what is suggested by the classical picture by Wells, for 50% relative humidity. With increasing ambient relative humidity the extension of the lifetimes of the small droplets further increases and goes up to around 150 times for 90% relative humidity, implying more than 2 m advection range of the respiratory droplets within 1 sec. Employing Lagrangian statistics, we demonstrate that the turbulent humid respiratory puff engulfs the small droplets, leading to many orders of magnitude increase in their lifetimes, implying that they can be transported much further during the respiratory events than the large ones. Our findings provide the starting points for larger parameter studies and may be instructive for developing strategies on optimizing ventilation and indoor humidity control. Such strategies are key in mitigating the COVID-19 pandemic in the present autumn and upcoming winter.

中文翻译：

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延长气雾中呼吸液滴的寿命及其对空气传播疾病的影响

为了量化在不同环境相对湿度下呼吸液滴的命运，对典型呼吸事件进行了直接数值模拟。我们发现，因为小液滴（初始直径为$10\mu \mathrm{米}$）被排出的湿粉扑中的湍流涡流所扫除，相对于50％的相对湿度，其寿命比威尔斯经典图片所建议的寿命延长了30倍以上。随着环境相对湿度的增加，小液滴的寿命进一步延长，对于90％的相对湿度，其寿命增加到150倍左右，这意味着呼吸液滴在1秒内的对流范围超过2 m。利用拉格朗日统计数据，我们证明了湍流潮湿的呼吸粉扑吞没了小液滴，导致它们的寿命增加了许多数量级，这意味着它们在呼吸事件中的运输比大事件要远得多。我们的发现为更大的参数研究提供了起点，并且可能对制定优化通风和室内湿度控制的策略具有指导意义。此类策略是缓解当前秋季和即将到来的冬季COVID-19大流行的关键。