The role of relative humidity and temperature on the dispersion and evaporation of pathogen-laden droplets: A LES-based study

Droplet evaporation plays a critical role in the transport and exposure to respiratory aerosols in indoor environments. This study uses large-eddy simulations within a Eulerian–Lagrangian framework to analyse and quantify how coupled variations of relative humidity and temperature affect expiratory particle dispersion. Three outdoor/indoor climate scenarios – winter, summer, and tropical – are examined, each involving the release of a polydisperse distribution of water–NaCl saliva particles emitted during 60 s of speaking. Droplet evaporation is modelled using a hybrid approach that applies mass and heat transfer equations for particles > 10 μ m , and a modified D 2 − L a w for smaller particles following Dalla Barba et al. (2021). Evaporation is stopped once the salt solubility limit in water is reached. An additional two-speaker case under summer conditions is also analysed to quantify jet interaction effects. Results show that identical droplets evaporate 30% and 100% more slowly under summer and tropical conditions, respectively, compared to winter. Slower evaporation in warmer, humid environments leads to greater retention of larger aerosols within the breathing zone and a more horizontal jet trajectory prior to buoyant rise, resulting in a 4.5%–30% increase in aerosol volume fraction at breathing level. In the two-speaker configuration, enhanced jet interaction produces a more symmetric dispersion and a 17% increase in local aerosol concentration. These results demonstrate the importance of incorporating realistic environmental variables when evaluating exposure risk to infectious pathogens in indoor settings.