  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
10um) 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 William F. 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 two meters advection range
of the respiratory droplets within one second. 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.
