An Impactor-Based Aerosol Platform for Probing Indoor, Short-Range Transmission Dynamics: A Phi6 Bacteriophage Study

Short-range airborne transmission is an important driver of the respiratory disease spread. However, the early postemission recovery of viable pathogen-laden droplets (VPLDs), relevant to close-contact exposure and transmission, remains poorly understood. Using an impactor-based platform, we assessed how solute composition, pathogen load, and indoor relative humidity (RH) influence postequilibration VPLD counts within a physiologically relevant aerodynamic range. Clinically relevant titers of Phi6 (1 × 105 and 5 × 105 plaque-forming units/mL), a SARS-CoV-2 surrogate, were suspended in proxy solutions reflecting airway solute content in healthy (1.5% w/v) and disease-relevant (3–6% w/v) conditions. Droplets equilibrated under indoor winter-like (15 ± 5% RH) or summer-like (50 ± 5% RH) environments. At low viral titer, recovered counts of VPLD (1.73 to 20.28 μm) were significantly higher under low-solute/low-RH than under high-solute/intermediate-RH conditions. Conversely, the solute content and RH did not significantly alter recovered counts of VPLD within the largest fraction (18.65 to ≥20.28 μm). At high viral titer, solute and RH conditions did not alter recovered counts of VPLD within smaller fractions (1.73 to 13.62 μm); however, within the largest fraction, recovered counts increased significantly under high-solute/intermediate-RH conditions. These findings suggest that droplet microphysics, host physiology, and seasonal humidity variations indoors influence the postemission recovery of VPLD.