Respiratory drop dynamics: Evaporation, sedimentation, and implications for airborne disease transmission

Hypothesis: Understanding and predicting the dynamics of respiratory drop evaporation and sedimentation are critical for addressing airborne disease transmission and devising effective prevention strategies. Existing models often face limitations due to the complexity of the evaporation process, oversimplified assumptions, or incomplete knowledge of respiratory fluid properties. • Theory: We present a novel theoretical model combining gravity-driven sedimentation under the Stokes flow approximation with a quasi-steady, diffusion-limited evaporation framework in a non-isothermal gas phase. The model incorporates essential respiratory fluid properties, including the sorption isotherm and solute diffusion coefficient, and is validated against experimental data. It applies to drops with radii <50μm, which are relevant for airborne transmission, and provides compact analytical expressions suitable for direct prediction and integration into larger-scale numerical models. • Findings: The model elucidates the physicochemical mechanisms governing drop dynamics, considering the effects of drop size, ambient conditions, and solute presence. Sedimentation times are predicted across a wide range of ambient conditions (relative humidity 0–100%, temperature 0–40∘C, and pressure 0.5–1 atm), emphasizing the critical role of environmental factors in transmission risks. We show that, over this full range, respiratory drop dynamics can be effectively described using a simple, analytical model based on pure water equations, complemented by the final drop size derived from the sorption isotherm. The model can also evaluate evaporation-induced pH changes resulting from ion concentration variations.