Effects of temperature and humidity on the spread of COVID-19 via respiratory droplet transport

Ever since the World Health Organization proclaimed Severe Acute Respiratory Syndrome-2 (SARS-CoV-2) as a pandemic in 2020, several research groups have investigated the correlation between temperature and humidity on COVID-19 spread. As respiratory droplets are expired into the ambient environment, they evaporate as they travel. Thus, most studies incorporate a droplet evaporation model. However, most models have not been thoroughly validated in terms of temperature and humidity. Herein, we first validated an evaporation model by imaging a droplet suspended in air using an acoustic levitator, placed within a constant temperature and humidity environmental chamber. Next, simulations were conducted using this model, to assess trajectories of cough droplets ejected from the mouth. Our results suggest that temperature does not have a significant effect on droplet trajectories, while humidity does. For larger droplets, higher humidity causes them to fall quicker to the floor and travel less. On the contrary, for smaller droplets, higher humidity causes the droplets to travel farther and end up at a lower height before evaporating, reducing the concentration of small aerosols near the respiratory tract. Therefore, we conclude that higher humidity helps in suppressing virus spread.