Physicochemical Diversity of Cough-Generated Respiratory Particles Revealed by Single Particle Analysis

The size, morphology, and chemical composition of exhaled human respiratory particles influence their aerodynamic behavior and their potential to carry and transmit infectious agents. However, the physicochemical properties of these particles remain poorly characterized on the single-particle level. In this study, we analyzed human cough particles exhaled by seven healthy adults by using scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM/EDS). This study provides direct, single-particle evidence of chemical and morphological heterogeneity in cough aerosols that is not accessible from bulk aerosol measurements. Cough particles exhibited a bimodal size distribution consistent with emissions from both the lower and upper respiratory tracts. Principal component analysis followed by k-means clustering revealed four chemically distinct particle classes, based on particle number: 54% carbonaceous, 3% Cl-rich, 16% P/S-rich, and 27% mixed salt. Area-equivalent volume fractions showed a different pattern with carbonaceous, Cl-rich, P/S-rich, and mixed salt particles contributing 27%, 41%, 1%, and 31% of the total area-equivalent volume, respectively. Each group showed characteristic differences in size and morphology. Cl-rich particles were larger and less spherical, while carbonaceous and P/S-rich particles were smaller and more spherical. Elemental correlation patterns, particularly among Na, Cl, K, and S, suggest heterogeneity in the particle origin along different regions of the respiratory tract. The distribution of particle types varied across individuals, indicating interindividual heterogeneity in the respiratory fluid composition. Together, these findings demonstrate that cough particles are chemically and morphologically diverse, forming heterogeneous microenvironments with important implications for pathogen stability, hygroscopic growth, and airborne transmission.