Evolving droplet dynamics during violent expiratory events at various ambient environments

In this paper we use simulation techniques to provide new insight into the effect ambient temperature and humidity have on the evolving size and temporal dynamics of the droplets exhaled during coughing and sneezing. Four different temperatures were investigated corresponding to various weather conditions ranging from moderate (T=16C and 25C) to more extreme conditions (T=4C and 45C with a relative humidity of 30\% and 60\%). Numerical observations show that regardless of the ambient temperature, the travelled distance of the droplets in sneezing is longer (2.5m) than in coughing (2m). Current WHO guidelines indicate 1.8 m social distancing which might not be thus adequate. We demonstrate-for the first time- that evaporation depends on the gradient of the exhaled air temperature and the surrounding air rather than the ambient temperature itself. Larger droplets are more affected by the ambient conditions since due to gravity leave quicker the exhaled air cloud. Smaller ones are trapped longer to the exhaled air and evaporate quickly due to body temperature initial conditions. An additional important finding is that in both events, the droplet clouds evolve towards lower Stokes numbers supporting the hypothesis for increased aerosol transmission risk as time proceeds. Our work contributes to the better mapping of the droplet sizes and transmission route offering information hard to acquire experimentally for their dynamics in different times. These results help to guide public health policies to reduce the transmission of respiratory diseases, confirming the importance of social distancing to prevent them.