Modeling Protein-Salt Breathe-Respiratory Aerosols for Early Dispersion Relative Humidity-Driven Evaporation Indoors

Airborne transmission of respiratory diseases is strongly controlled by the evaporation and transport dynamics of exhaled droplets, which are governed by both ambient relative humidity ( R H ) and the complex non-volatile composition of respiratory fluid. Traditional models have frequently oversimplified this process by representing droplets as pure water or with a single, uniform hygroscopicity parameter, dismissing the roles of proteins and salts. In this study, a novel Computational Fluid Dynamics ( C F D ) framework is developed, coupling a turbulence-resolving Eulerian–Lagrangian solver to a κ − K ö h l e r -based evaporation module that rigorously accounts for protein and salt solutes under realistic indoor R H . The model quantitatively reproduces experimental hygroscopic growth data, predicting equilibrium droplet diameters within 3 % of reference values across 20 − 85 % R H . Results reveal that salt-rich droplets retain up to 45 % larger equilibrium diameters and evaporate substantially more slowly than protein-rich droplets at low to intermediate R H ( 20 − 53 % ), while compositional differences vanish at higher R H ( ≥ 70 % ). A quantitative investigation of simulated respiratory droplets pronounced differences in dispersion and airborne persistence compared to equivalent pure-water or non-volatile analogues, with composition and RH exerting non-monotonic, coupled impacts on transmission-relevant behavior. These findings bridge the gap between idealized and reality-based aerosol models, demonstrating that multi-component evaporation modeling significantly enhances the predictive power of indoor transmission simulations and provides a practical basis for humidity-targeted infection control policies.